Hello rankaudio,
I'm glad you're so pleased with the tubes driving the Maggies but there's still quite a bit of bass on your sources that you're not hearing. Humans are capable of hearing bass tones down to as deep as 20Hz and we're unable to determine where bass sounds are coming from when they're at about 80 Hz or lower.
Because of these facts, almost all recorded music is summed to mono from about 80 Hz and lower while only extending down to 20 Hz. Only some hi-resolution recordings even record bass below 20 Hz, which is bass that cannot be heard but can be felt. Fortunately, there are very few musical instruments that are able to produce bass below 20 Hz (pipe organs are the only ones I'm aware of) and most mass market recordings filter out all frequencies below 20 Hz.
The Magnepan .7 speakers are rated to have bass output down to 50 Hz and the REL T5i subs are rated to have bass output down to 32 Hz. So, with the subs installed you'll extend the bass from 50 Hz down to 32 Hz, which you will likely perceive as significantly deeper bass response, bass impact and viscerally more realistic and enjoyable. With your current choice of subs and main speakers, however, I want you to be aware that you'll not be hearing any of the deep bass tones on your recordings from 20-32 Hz.
This omission can be significant in the realism perceived and the visceral bass impact heard and physically felt, the degree to which this will be missing is largely dependent on the type of music you are listening to. I find that the accurate reproduction of the deepest bass octave and bass dynamics results in my very realistic perception that the musicians are in my room or that I'm listening live in person at the recording venue. I listen mainly to a wide variety of rock, acoustic and jazz music.
Nevertheless, I believe your combination of midrange and treble frequencies being reproduced by tube electronics driving the .7speakers from 50 Hz on up and the bass frequencies being reproduced by the pair of solid state amps in the REL T5i subs driving their 8" woofers from 32 to somewhere between 50 and 60 Hz, is capable of being a high quality system provided the speakers and subs are positioned and configured properly.
My recommendation is to continue to operate the .7s full-range, position each REL sub sequentially using 'the crawl method' and then adjust the volume, crossover frequency and phase controls on the subs to attain optimum bass response and a seamless integration with the .7s in your room. Whether you perceive the bass as sufficiently deep, accurate, detailed, natural and realistic for very enjoyable listening experiences is a matter you will need to determine.
As I stated, I really enjoy the accurate reproduction of the lowest octave in my system. But this may not be as enjoyable or important to you. If it's not, your current system should prove very satisfying to you. If you prefer deeper, more impactful and realistic bass, I'd suggest utilizing a pair of larger subs with larger drivers and more powerful amps, the $3K Audio Kinesis Swarm or Debra 4-sub DBA system, or even a custom 4-sub DBA with your subs of choice with 10" or larger drivers.
Best wishes and enjoy,
Tim
