What is wrong with a sub?

@sgordon1 thanks for the support. To answer the first part, I do not have the time, or the money to test every combo. In cases where I do have the money, I’d like to know what others think hoping to put it all together and do it right. I have a set of speakers that my amp does not drive well. So I ordered another amp. Fun, costly and somewhat irritating. Maybe a sub would be the trick - too late, I ordered the amp.

Had I known how much I prefer the feel of speakers and sub over headphones I would have saved money and time there too. I didn’t need to spend what I did on head phones. A sub helps with that in my limited space. It also seems my speaker sub pair might sound better than what I can do for a few K more.

The comments seem to be supporting what I thought.
Taking this to the $2,000-$25,000 speaker range for music that has lower bass in it, for the same about of cash...
Equivalent or better sound will likely come at a lower cost with speakers and a sub, than buying speakers alone.
AND
Many speakers build low range drivers in them, and buying the better mid-range speaker paired with a sub is likely a better value.  

FWIW I enjoy tube amps and lower power near field.  So I would not be buying a high watt SS amp to pair with some Maggie's which I'm told are great.  I had to choose between more expensive larger speakers with low range or going with a smaller speaker and good sub.  I did the latter, but am buying more stuff and wondering if I should change my thinking for a larger room.  I plan to do the same.  Good larger speakers, a sub to fill out the lower spots.  Music is Bach/classical (I don't hear the sub) and some pop/Fleetwood Mac kinda stuff where I certainly do.

@mcreyn - that was a bit Greek to me.  
What I have now:
300B SET (CAD-300SEI) with Watkins Gen 4 speakers on each side of my desk 8" high stands, REL i7(? small one) sub under desk to my left. 
The sub hooks to the amp speaker posts.  I'm pretty satisfied.

New amp is A/B 60W/channel and Vandersteen 2ci (which the SET amp can't push well).  This will go in a larger room with cinder block walls.  My plan is to get a larger REL sub and again hook to the speaker terminals.  I was also thinking Klipsh Forte instead.  Some of the Martin Logan looked good, but were pricey and had subs built in.  That kinda got me thinking why not separates (subs).

@mkgus, thanks for that.  My one REL i7 I rarely hear.  Sometimes I do mess with the volume on it and pump it up beyond the music levels, but normally just because I want to wake everyone up.  

Got me thinking if 2 and 4 are better, is there any reason they should all be the same?  Could you mix an i7 with i9 or other brand?  I would tend to shy away from mixing brands.  But when I think of "real" bands and or orchestra there are different low end sounds of different power coming from different locations.  Maybe they balance those, just the base drums are not always center.  

I don't know what DSP is relative to subs.  I assume this? 

I'm too new and inexperienced to take a side.  But as I posted - I like tube amps.  I like how they sound.  As such at reasonable price range tube amps there are no ribbon speakers I can use.  The subs having their own amps move the air fine.  I also like the argument that the position of the subs may not be where the mids are.  

I bought a smaller REL i7.  I was thinking maybe I should go bigger. 
Assuming the same total $pend...
Seems several think having several smaller might be better than one bigger.  I expect bigger can go lower Hz.  If this was for a home theater, maybe I'd get the earth quake / bombs better with a single bigger sub, but for music, which is what I want, that little i7 REL is low enough - I think, then I don't have the bigger ones.

Thoughts on more smaller vs one bigger?

Using my brain, not my ears, seems that the lower freq are just fine located in different locations.  I would not buy a full range speaker until I had this figured out.  If the speaker is just more cone speakers of different size and the bottom one is big - why not have it located someplace else?
 
In live music the instruments are in different spots.  Bi-symmetrical (stereo) may be because we are bi-symmetrical with two ears.  However lows are both felt and heard.  I feel a sub which is why for me speakers sound better than headphones.  I'd rather move the sub around. 

Guess the benefit of being old and new (to audiofile stuff ) is I have not had that issue, which was the reason behind the OP.