your opinion on subwoofers in two channel

I think good subs that can be tweaked in to mate with your room & system do make sense and can add to the enjoyment in a meaningful way. Less adjustable subs or trying to do to much from a single sub can be more of a distraction than a benefit. And it depends on your taste in music. If there is little to be heard then a sub adds little (duhh, ok so that ain't such an epiphany).
To answer your question directly, if I had to choose between better sound overall or adding a sub, I'd take the former, but if I could afford the additional extravagence of one or two good subs I'd go for it. IMHO, YMMV, DEYS.

Can be a good thing if you like the sound of your system for 2 channel music and just wish it had more sub bass. You can enhance the lows without messing up the rest of the system. Also if you don't have the room for large floor standing speakers.

If the smaller speakers have great mids and highs, a really good sub can make them sound like larger speakers costing much more than the cost of the 2 speakers and the sub if you can get them to integrate well. REL subs are pretty easy to integrate.

Back in the 90's a lot of people would match two of the small B&W Matrix 805 speakers with the Matrix 800 sub. It would come close to the performance of the Matrix 801s for those with smaller rooms and/or those who could not afford the M801s. A pair of M805 plus the Matrix sub was about half the cost of the M801s. Probably is still the case with the current B&W 805s and subs.

Sugabrie is right but prices are pretty high,Wouldn't go with a B&W sub or Velodyne etc for music.REL,Martin Logan etc get excellent speed and pitch accuracy by using multiple small drivers.I think that the Vandersteen 2weQ is oine of the best valuies in High fi$800 for one used $1200 (?) new.Has three small drivers that are muiscal where ars unless you spend meha bucks on large driver sub it will be too slow.Cool thing about Vandersteen as well is it's x-over which is designed for you to hook wires together with your satelites at speaker posts'X-over "see's" whole waveform of muisc not just what is low passed under 150 (or what ever) Hrz and results a a suipperior product at a bagain price.Never was big fan of their well selling 2Ci m(* though big ones are good and costly) but subs are so good that many folks get and at the price end up with a apair.I knwo it shouldn't make sense because you are not supposed to localize bass beneath 150 hrz and therfore only need one but hoowk two of them up and you won't want to give it up.NHT made a good two chanell sub.Depending on your speakers subs are worth it oenor two remeber but way to many folks foget the name is SUB woofer and turn them up to loud and set the crosover too high so they "hear what they paid for".You shouldn't "hear" the sub.It should provide a foundation.If you chosee a value amp don't get sucked into cable snake oil and hav smaller two way spoeakers you'll have better imaging than a larger cabinet.Some of the best b\valuies in Hifi seem to be the hybrid integrateds by companies lie Unison and Pathos.Digital get's better and better so my mind is save there and update that when finaces allow.
Chazzbo

Hi,, there can be a problem with full range speakers in some rooms because the proper spot for imaging etc may not be the best location for the speakers low frequency performance,,,similar to how a sub has to located in a particular for smooth response.

I second Butsy: full range floorstander may offer you a real placement issue between imaging and bass response (room nodes and standing waves). With a sub, placement is way more flexible and you get the advantage of monitor imaging. $ for $, I would think monitor + sub is a better deal unless you start spending over $7-8k on large full range speakers IF AND ONLY IF both monitors and sub are well integrated (you need a friend and some patience). In the budget category, Martin Logan - even the Grotto ($750 used) is a sweet deal, then REL. If you do not have monitors yet and have $5,000 to spend for both (price new), combo ProAc 1SC and their new sub. Amazing.