I have owned and still own ARC D-90B amps. The original D-90, used the 6550 output tube, as correctly stated by baddlci above, and based its inputs on the 12AX7 tube. The sound of the D-90B was vastly improved over the D-90. A complete change to 6DJ8 and 6L6 occured. There was a smidgen less authority vs the 6550 version, but the B version amplifier was a complete success in 3D, MUSICALITY in spades, utter naturalness and rightness in tone. The D-115 and D-70 series, was a significant step down in my opinion. Air and ambience retrieval seemed less. A sense of dryness entered the picture, and overall less enjoyment. The MK II version, of the 115, improved things greatly, but the 90B magic, was never fully recovered. Things of course went rapidly downhill, into the Classic 60 series of hybrids. ARC, would not recover from this misguided direction, until very recently. The prices today are just too high, for the circuits that approximate the older gear.
The great ones were,