Need recommendations for better two-channel audio out of my Marantz SR-6005 and my B&W 683

@georgepv , yes, the Rythmiks have way more DOFs (full 0 to 180 variable phase control, PEQ, etc) to help integrate a sub better than many other subs out there for 2 channel systems. Sealed/servo-controlled is the way to keep it precise and clean for music and they do dig very low at 12 to 14ish hz. It is a steal at this price point.

PM8006 has a poweramp direct in, which bypasses the integrated amp’s preamp section. It would be the exact equivalent of using your A23 power amp on its own with the receiver’s preouts. But, you also have the option using its preamp (a.k.a use it as an integrated amp in a dedicated 2 channel system). I was trying to suggest a "step by step" upgrade path in my earlier post.

But, since you mention that you already have a separate 2 channel system and want to use your 5 channels for multichannel as well (music and movies?). your best bet would be something like the Yamaha RX-A3080 receiver.
https://www.accessories4less.com/make-a-store/item/yamrxa3080bl/yamaha-rx-a3080-9.2-ch-x-150-watts-a/v-receiver/1.html
It is Yamaha’s current flagship (before they start rolling out the 8k receivers) and you can get a lower price on it these days. It towers over the Denon flagship and the Pioneer Elite flagship i have compared it against for MUSIC (It will walk all over your older gen Marantz receiver though i have not had a chance to compare it against Marantz’s current flagship). It is perhaps the best receiver and 1 box solution i can think of for 2 channel and multi-channel music without emptying your wallet completely. I have a significant collection of multi-channel SACDs and blu-ray audio i listen to with this receiver (in addition to 2 channel sources). It has been stunning for the past couple of years! Without stating the obvious, it would be a no brainer for your movie collection as well.
A.Robinson just recently reviewed it here.
https://www.youtube.com/watch?v=fBifGxSda7s

One thing to be wary of..... the RX-A3080 is jam-packed with enough features to overwhelm a multichannel noob. But it is well worth the upgrade once you study the manual, ask questions and figure out how to work it. In other words, it may overwhelm someone who’s buying a receiver for the first time. But, it shouldn’t be a problem for you, since you have dealt with receivers before.

Best,

Agree with @deep_333 that the Yammy Aventage AVRs, especially the upper models (1080 on up) that have a power transformer are very good. I worked for Magnolia for a spell and got to compare Denon, Marantz, and Arcam driving B&W 804d3s and thought the Yammy outperformed them all in 2-channel. Sound was detailed, transparent, refined, and imaged better than all others with a clearer background and a superior 3D soundstage. I was pretty surprised by this, especially regarding the Arcam. A customer who owned a Marantz AVR had me compare with an Aventage AVR, and let’s just say he was no longer happy he owned the Marantz.

That said, you can try subbing in your A23 and I’m sure you’ll hear an improvement, but you’re still stuck with a mediocre-at-best preamp section in the AVR that will continue to be a significant impediment to better 2-channel performance — hence why we were recommending using an integrated amp. If it’s me that’s what I’d do rather than switching AVRs. I’d think adding something like a NuPrime IDA-8 or the like would yield a huge improvement all around for not much $$$ (probably could use its DAC too). Anyway, hope this helps at least somewhat.

I ended up getting a Denon X3700H. Since this is mainly a home theater setup I wanted to stick with an AVR. After running through the Audyssey setup it sounds great. The speakers blend together much better in surround sound and the two-channel sound is significantly better as well.

This AVR also has two subwoofer outputs so I can add dual subs in the future once I refill my slush fund. This unit was a bit pricey but I am very happy with it. It gave me the level of detail I was looking for in two-channel and boosted my home theater as well.