Mark, welcome. Usually my preference is bookshelves plus sub, but your GTP400 is a basic two-channel pre-amp with no provision for bass management. If you stay with it you won't be able to take best advantage of a bookshelf/sub combo without additional processing to crossover at e.g. 80Hz; the bookshelves would run full-range. So, the better move in my view would be to go with the M60s if the budget can be stretched or the M50s if not. With your 15' longest room dimension, room gain beginning at about 38Hz will give a theoretical 12dB per octave bass boost, which should result in good bass down into the mid 20s. This should be very good with nearly all music(pipe organ 16 Hz pedal excepted)although not as impressive with explosions and other LFE in movies as a good sub would be.
Long-range, even for an all-music setup, you might consider multi-channel equipment which can process(e.g. DPLII, Logic 7)two-channel source material and send surround ambience to surround speakers such as the QSs, where it belongs, with a resulting increase in realism.
