Where do I start!?!

Ask your FIL for advice....I'm sure he'd like to help you with setting up this system.  He obviously has the knowledge and can suggest the pre-amp you'd need.  You'll just get too much different advice here.  Keep them!

To the OP. With the gear your father ( not father in law ), has graciously and lovingly given you, you should feel special, and lucky. With that, you need to do your own do diligence, and research the amps and speakers, to determine what Mcintosh had in mind, for the specific purchaser, of those components. You are getting a lot of information from others here, and I disagree with much of it ( not all ). Is this 12 X 13 area, an enclosed room, with a single door, or is this area open to another area of the house ? If this is a room, I believe, ime, this Mac gear will over pressurize the room, with not only bass issues, but time alignment issues with the towers, creating " too much " of everything. These are not, by any means, near field speakers ( close up listening ), and I totally agree with caphill on this. Again, if this is a room, with 4 walls, there is no amount of acoustical room treatment that will get you to a very good listening experience. As far as the gear being accepting by you, from your dad, it is obvious he does not need the money for the gear, but I would consider keeping it for your next move ( a larger space ), or graciously tell your dad, to sell it. And cables...ridiculous, at this point. People recommending other gear to you, as far as Naim, and other speakers ? You have not mentioned what kinds of music you listen too; you have not specified if this will be a dedicated 2 channel audio room, or a combo home theater / audio room. I do not want to think you are a spoiled rich kid, as I have known, and helped many, with setting up systems. I am a retired audio consultant, and feel, with your intelligence ( your writing is very mature and articulate ), you can figure out if this gear is for you, and your room, and go by that. To much " personal " opinions here, as it is easy for people to recommend what " they " like. I am sorry if I sound a little harsh, but my sciatica had me in the hospital overnight, so I am a bit tired and cranky. Every one here means well, as I feel A’gon has a wonderful group of passionate folks, dedicated to music, and the equipment to get them to their nirvana. Also, asking your dad for anything else at this point, would be selfish, as he already indicated that " you " need to find a preamp, etc, to complete the system. The Thorens would be a go. The fact that you posted this, I am not sure you are ready to own gear yet, at this level. I know this is all a bit of mumbo jumbo, but I know the medications I am currently taking, are effecting me, in many different ways...so I am going with that excuse. Hope everything works out for you, and sorry if I insulted you, or anyone else here. It was not my intention. Enjoy ! MrD.

+1dentdog
And agree 1000% with Elizabeth. 
You are one blessed individual!!!

I would start looking around for a new abode.

Don't even consider selling any of that stuff! What a lucky soul you are....
and do consider the Schiit Freya, would likely be a beautiful match. Or a used Bryston pre. Better yet, a used Musical Design pre (if you can find one).

I wouldn't sell it, if for no reason other than family harmony.  And you owe your FIL one BIG Christmas gift this year!!! Enjoy what you have, and invite your friends over. ;-)

In that small listening room (12×13) I assume you are only doing 2ch (stereo) for listening to music only. If you’re only doing stereo why in the world did you end up with a Denon AV receiver in the first place? Just get a very good quality stereo integrated amp and a pair of standmount (bookshelf) speakers on stands.

Your father’s McIntosh MK 1.2 monoblock amps & preamp & massive McIntosh speakers are overkill for your listening room size. Those kind of gears are meant for large listening room. Sell everything all McIntosh gears, your Denon AV receiver & your Monitor Audio Silver speakers and get a high quality stereo integrated amp and good quality pair of standmount (bookshelf) speakers for your small listening room. That’s all you will need. There are countless options out there.

For that such small listening room I would highly recommend getting a Naim Uniti Nova all-in-one box solution. It is a digital music streamer/DAC/integrated amp all in a single chassis. equipped with built-in Tidal lossless music streaming app but no built-in phonostage. It will be best to acquire a very good external standalone dedicated phonostage pre for a turntable. The Naim Uniti Nova retails for $7500 brand new. Naim also makes a dedicated phonostage pre and you can connect an external phonostage pre to the Naim Uniti Nova’s linestage analog inputs. The Naim Uniti Nova integrated is a highly musical sounding integrated amp/streamer/DAC unit. It has various digital inputs and analog inputs (XLR & RCA) & DIN (Naim’s proprietary connection). You can stream Tidal directly and download digital music files. I highly recommend getting a Naim Uniti Core dedicated music server to store all your digital music libraries as it would make a big difference sonically rather than using a NAS or central server or your computer to store your digital music files as NAS or your computer are inherently noisy. And connect the Naim Uniti Core music server to the Naim Uniti Nova. 
I highly recommend the Naim Uniti Nova! It is compact in size and sounding highly musical. Its onboard streamer/DAC is superb. 

For a pair of standmount speakers I recommend PMC or Focal Sopra 1 or B&W 805 D3 or ProAc or ATC or Sonus Faber Olympica l standmount or Audio Physics to pair with the Naim Uniti Nova.
PMC, newer Focal, B&W, ProAc, ATC, Audio Physics, Magico speakers will pair really well with Naim electronics and would have great sonic synergy together.

For my bedroom setup I personally have the Naim Uniti Nova paired with PMC standmount (bookshelf) speakers. Sonically it is perfect match made in heaven. They have great perfect synergy together. Prior to PMC, I had the Uniti Nova paired with Sonus Faber Olympica l standmount as well as B&W 805 D3 standmount. They also sounded great together.

I'm not sure why you think selling off something will require him to buy some "Junk" Elizabeth. regardless of the prices involved, as that's irrelevant as all used equipment has taken the depreciation hit. 

Who's to say he can't trade in his mono blocks for another lower powered stereo amp and preamp (with a phono section) from McIntosh or a top shelf integrated with similar sound quality.  

Also that trade in money would go a long way to optimizing his whole system and room treatments. 

you have some good points though about trying it first and if that's a possibility go for it. 

in the end he will have to spend money to get the system working up to par with room treatments and a proper preamp with phono stage of similar quality (or what's the point). lets not even get into wires-cables-stands and racks etc. 

This is why I feel he's better to trade-sell and start a system approach rather then try to make something work in his room that was designed for a larger space.  

Why not try it out?  And see what you think you may be surprised.  I would recommend to wait to sell them.  You will lose a lot of money especially on the speakers.  You may be happy with it, and then you can tweak the room or system components accordingly later.

You definitely need to pair the amps with a similar level of preamp to maximize the performance.  I would try to get a McIntosh Preamp since that's the only thing other than the turntable that wouldn't be McIntosh.  System synergy is important.  McIntosh tube preamps tend to match well with SS amps.

Cables will be very important in this system.  Can you get cables from you father to try out?  The only way to really know what cables work well is to start somewhere and then see what you like or don't like.  I am pretty sure the 1.2KWs are Quad Balanced so theoretically balanced/XLR cables will work better, but if you have RCA it should work great as well.  Just a word of advice especially using RCAs, the 1.2KWs have enormous amount of power, so never touch the any of the interconnect cables when the amps are on.  You can blow the speaker with the simple "pop" sound that can occur.

I have to agree with a few here talk to your father and see if he minds you selling the equipment and upgrading to a nice integrated amp -  smaller separates and speakers. I hope he will understand that your just optimizing the system to your room and not wasting a gift. 

 I've lived in a room similar to yours and as much as people would like to say larger systems will work they really wont work optimally in that size room, your better to down size at least the speakers to a small floor stander or monitor and you will get closer to the full potential out of your room speaker interface. Room treatment is going to be a consideration for sure. 

I tried to run Proac Response 2.5's years ago in a room very close to the size of yours and they just did not work well. I had to put so many bass traps and other room treatment just to get it ok but never great. 

Regardless of near field or not you will be better to optimize the room speaker interface with an appropriate sized speaker for the room. 

Another consideration is speakers designed to be close to the wall or in the corners. AudioNote AN series comes to mind there are others, but that will optimise your space much better.  

glen

You hit a JackPot in terms of Sound Reproduction / Gear.
A Line Array!
Hard to beat in any size room!
A MacIntosh Rep / Dealer would likely recommend a product from 
the Mac Line.
Look into these in order to form some idea of what the gear is designed to do.
A Preamp with Integrated Phono Pre could work well.
Very well in fact.
Any Component Preamp from Passive to SS will give a good idea of Sound.
Finding gear is a big part of the fun!
Heck ... the Agon Market should give a good idea of this range.
Please do not even think about selling until you have an idea of what you have.
Experincing the Power Meters barely budge while hearing an Ethereal 
Playback of a Demanding Passage from a Classical Recording will convince you. 

That room is going to require some serious acoustic treatment with expert advice, especially with that gear. 

I agree with Elizabeth.  I also assume that selling the equipment your FIL just kindly gave you would be an insult.  

You will need some room treatment, especially behind the speakers as the speakers will closer to the rear wall than optimal.  GIK Acoustics makes some great diffuses and absorbers for very reasonable prices and will advise you as to your best treatment options.  Figure $500 to start. 

Once you look at a preamp, going balanced, your options start at $699 for a Schiit Freya S (an absolute steal at the price and a good preamp at any price) which you can add a Mani phono preamp for $129, and they go up from there.

Don't go spending a bunch of money on cables right now.  Blue Jeans makes great cables at fair prices.  For balanced, I like the "worlds best cables" on amazon made with Mogami wire (they also make great RCA's with Canare and Mogami).  Zu Audio also sells their older cables on Ebay for good prices.  Cables are like religion, but even the most ardent cable believer will tell you, Cables are the last step in the system after you have dialed everything else in.  

ENJOY.  Your FIL has given you a wonderful gift.  

What is your budget?

Tough spot.  You are emotionally invested and really have no viable real estate to wrestle this monster into.  Recommendation would be to sell the mono blocks and the speakers as they overwhelm the space and cannot sonically optimize it.

If you like the Monitor RS6 then keep them.  The Denon really is weak for two channel.  Run the system with a nice intergrated with phono.  Best of luck

Most current preamp products have both XLR and RCA outputs.  Get something like the PS Audio BHK signature pre and you are fixed up.  There are a number of these excellent units on the used market at present at around $3k.  Just remember to have fun while figuring it all out.  You are embarking on a fantastic journey of discovery.