I tried both the below solutions to some brightness issues with our floor model JBL Everest DD66000's just over the last two days. Here is what you can expect.
When passively bi amping like you plan, you will need two amplifiers, regardless of whether tube or solid state, that have the same db of gain. Otherwise an amp with higher gain placed on the horn for example, will make it louder than the woofer. The would be the opposite of the effect you want. You may have to call McIntosh for this specification as I would not assume that all MAC amps have the same gain. Input sensitivity, measured in volts, is also a specification that you can rely on. If the numbers are the same for equal power output, the gain will be the same.
The problem with the bi amping approach is that the brightness is almost always a frequency response error with either the recording, room or speaker. With bi amping you would be trying to correct this problem with the more forgiving character of a tube amp, which does not change the frequency response significantly. It likely will soften the leading edges of transients a bit and warm up the sound overall. But unless your existing amp is glassy or sibilant sounding by nature, the bi amping is unlikely to go far enough toward solving your problem.
Have you consider room correction or graphic EQ? If the brightness you complain about is intermittent based on the recordings, you may just want some EQ or even basic tone controls to make these recordings more listenable. However if the problem is caused by frequency response errors generated by the room or the speaker itself, you may want to try the new McIntosh room correction equalizer or something like it. The cost is about the same as buying a MC-275, the $4000 range. Personally I think this much better money spent as you will solve a host of problems at many different frequencies and very likely the brightness problem too. Another consideration is upgrading your preamplifier to the new McIntosh C-50 preamp coming out next month. It has seven bands of graphic EQ built in, perfect for program material correction and also some limited room or speaker correction. Hope this helps. John.
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