a few points back to you
1) my comments on a bit brightness/brashness/harshness from the belles aria refers to my experience with the aria only -- i have kept in my stable of amps my beloved belles sa30... yes it is a little bright, but silky smooth (zero harshness or grain), utterly refined... not enough power to push maggies though...
2) if you have had great experiences with belles, maybe try the aria signature model? i’ll bet the sonics are a step above the regular aria, more in line with david's better power amps over the years
3) i very much believe the path to happiness with the .7’s is to use the rel sub pair in concert with them, well placed, using high level inputs, crossover around 70 hz - this will make more a sonic difference to any change in amp over a belles aria -- also, do you have the .7's on proper stands (magna risers)?
4) audio research is indeed a lovely lovely match to maggies but one must deal with the expense and the whole ’experience’ of running a high powered tube amp....and i would submit the i-50 may run out of juice even in a modest room if music is played to reasonable volumes (90 db peaks)
5) for 5 grand you have a lot of excellent choices -- ! i would direct you in the direction of pass labs, upper odyssey, hegel h390/h20/590, you can pm me i can provide some other 'sleepers' .... otoh all but the more expensive upper model ayres are a little too relaxed/refined to drive the maggies in experience...
good luck and have fun