With 6 + 4 panels, you may have the big daddy of the Tympani’s---the T-III. A rare beast, congratulations! If not, what are they---T-I’s or T-IV’s with extra bass panels? By the way, the mid-bass produced by Tympani’s (and bass down to almost 30Hz if the panels are braced) remains the most life-like I have ever heard, though Apogees---which I have never heard---are reported to also do bass very well. I gotta ask you: how are you running all those panels without bi-amping them?!
Are you currently satisfied with the D-250 in regard to midrange and highs? The solid state D-400 and paired D-100’s are better used on the bass panels, if you want to keep an all-ARC set-up. Why not, it’s a classic! The D-150 remains a particular favorite with ARC enthusiasts, as does the SP-10 (Brooks Berdan owned that combination until the end), but the D-250 is no slouch. So if you’re happy with the D-250, just add whichever bass-panel amp you prefer. Or, put the D-150 on the m/t panels, the D-250 on the bass. Those bass panels can eat as much power as you throw at them; Satie over on The Planar Speaker Asylum feeds his T-IV bass panels 2000 watts a side!