why use towers if you've got a sub??

Great question and I'd be surprised if there weren't many different opinions and even more forthcoming.

"Textbook" in a home theater setup, you are likely right. However, there's always an exception and let me explain my perspective.

In my opinion, with an LFE channel from the source, then there is really no big, huge advantage to having towers except that many towers tend to push more air. Thus, if you have a larger room, towers are better than monitors. Secondly, even with an LFE, 80 isn't necessarily the best crossover in your particular room. Yes, it's the THX standard, but sometimes you get a better/flatter response by crossing over at 60-75, depending on the crossover options in your sub or pre-pro.

(As an aside, studies from Floyd Toole show that the best setup in a home theater environment is to have 4 subs all at the mid point of each of the four walls. This provides the smoothest overall bass response in the room. These proven research runs counter to the "conventional" wisdom of one sub and in the corner. This model of four subs in the mid-wall points is now the accepted, ideal method for integrating subs in a setup.)

If you are watching TV or listening to stereo sources, then there is not necessarily a dedicated LFE channel. Thus, in many of those cases, the low frequencies are likewise stereo. You could argue that you can't "hear" the low frequencies, but I'd conjecture that you will get better integration top to bottom with towers. My example is simple here. I have Revel M22 mains and the B15 sub. Spectacular combination. I listened to the Revel Ultima2 Salons and the bass was so tight I was amazed there was no sub. That perfection of the crossovers with a tower speaker simply cannot be put into words. You need to experience it to understand.

That again goes with the previous poster about subs. My Revel B15 is a spectacular sub and handles both music and movies exceptionally well. The REL units as mentioned are also absolutely fantastic subs. Most subs don't get to level of the Revel or REL they get very sloppy. Thus, if you don't have an utterly spectacular sub, you can certainly get better overall performance by having towers and a sub.

I don't know ultimately what your goal is with your question, but my advice is to always try and buy the best quality you can within your budget. If I personally have a choice, I try and go with the higher quality speaker--even if it is a monitor--because you tend to get the better tweeter and midrange. Then you can complement with a stellar sub and have a truly magnificent home theater and music experience.

I use my HT setup as my 2-channel setup, so I can only comment from that perspective. My end goals are the same as yours.

I'm personally very happy and satisfied. I use an Anthem with ARC for both 2 channel and HT setup. My only comments on your thoughts are:

1) Don't skimp on the sub. Listen and get the best sub you can afford. It makes a huge difference. This is especially the case if your speakers only go down to 40-30hz. I'm very, very happy with the way my sub interplays with both music and movies. My Anthem allows me to have custom settings for music with the sub crossover, etc. and also totally different settings for movies. Thus, I get the best of both worlds.

2) I can respect your thoughts for a separate preamp just for 2-channel. If you have the cash, go for it. That's all a matter of personal preference. Don't know where you stand on room correction, but I've become a huge proponent since i've seen what it has done in my room. I spent quite a bit of time positioning and testing my speakers. Room correction took it to another great level as the room was playing an effect as all rooms do.

3) I would not skimp on the "basic" AVR (again, don't now what you're calling a basic AVR). If you're serious about your HT experience you'll regret going with a basic one

4) Unless you have an unlimited budget, I'd do one setup and spend the $$$ to find a set of speakers that will do your 2-channel and HT. There are many models and companies that do this and do it very well if not spectacularly well. NOLA, Revel, Focal, ADAM, B&W are but a few that I would strongly suggest you listen to. In fact, the NOLA speakers are some of the most musical and dynamic speakers out there that do an absolutely stunning job with 2-channel and also with home theater. The ribbon tweeter, and open baffle is to die for. The dynamics and also depth of stage you get is wonderful. If you can listen to the NOLA Baby Grands, your jaw will drop. Ditto with the Revels and the Focal Utopia series. You have plenty of options out there.

5) If cost is no object, then look at Audio Research products that will satisfy your tube itch and also be able to do HT and 2 channel with equal authority and grace.

6) i agree with your ideas about the quality progression from fronts to surrounds; however, try to stay in the same brand or speaker family. Otherwise you will notice a difference. I don't care what anyone else says, if you have a high-end setup, you will notice the timbre difference with different speakers from different manufacturers.

Finally, have fun.