A fun thread to read and lots of great ideas, products, technical information, and opinions. With years of enjoyable experience with MG II's through 20's including IIIa's with Entec and 3.6 and 3.7i with Rel Stentor III's subs as well as different amp and room scenarios here are a few key points to consider in chasing the extra octave.
I respect your position on not wanting to get into the details of amplication and room dimensions, however you talk about "you think you are missing a bit of low end" on some songs. You have to consider the reality of inefficient high resolution speakers like Magnepans and others. Like it or not, you will hear the strengths and weaknesses of up stream equipment as well as the differences in how the music was engineered and recorded. Sub's can easily create the feel it in your chest sensation, but as you said its not the experience you had in the symphony hall.
My point is, you have to decide on what you want - move octaves or move closer to the recording. With Maggie's what is up stream, room acoustics and speaker placement is how you get closer to the recording. Had lots of fun with my subwoofer experience, mostly the MG 3.6 with the Rel Stentor III, in an 18x25 room with carpeted cement floor with a few carefully placed room acoustical panels with the speakers 50" out from the back wall. Driven by CJ Art III and Levinson ML333 there was a big, dynamic and felt it sense to the recorded music especially well engineered and recorded music. It was different with 3.7i mostly due to the improvements in the 3.7i causing much tuning with the Rel's. Not a negative about the 3.7i's more the realization I was playing with an unnatural configuration. At least in my mind.
The change I made and would encourage you to think about is. Instead of chasing the extra octave, think about upgrading to the MG 20.7's and as JPMEURER2 stated - bring ample dynamic power over a sub system investment. I swapped out my ML333 for the Sanders Magtech mono's.
The sound is and feels more natural, especially with well engineered and recorded music. Don't forget not all recording are created equal and seriously play with speaker placement given available real estate over more equipment in the room. I sense many Magnepan owners do not move the speakers far enough from the back wall.
All the best.
