Making Stereo amps into Mono

One correction to your post, Paul. Nelson Pass did not design the Nakamichi amps. Nak simply paid licensing fees to Pass for letting them use the Stasis design. While i've never heard the Nak versions, i've been told that they sound QUITE different from the Threshold versions. Sean
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There was an article in Glass Audio, on how to build, I think it was called, a phase inverter. It was a inverter of some kind. It looked really simple, to construct, and it turned any stereo amp into mono. Glass Audio routinely sells back issues, so it would not be hard to find.So if you're deadset on going mono, that would be one way. I agree, though, that vertical bi-amping would be better. Once you get another amp, all you would need, would be more speaker cables.

Upgrading the power supply capacitors is another alternative. You could "bypass" (connect in parellel) the older supply caps eith newer and faster poly film caps. This will help make the older cap function more efficiently and will result a cleaner sound and faster dynamics.

Make sure you know how to do it. If unsure, please contact a local repairman.

Bad idea to bridge a stereo into mono. The tonal balance of the amp will change and will result in inferior sound. I would suggest you find another PA7 and use them to Bi-amp your speakers. This way, you power the lows and highs saperately....way better than to bridge.

Bridging not properly done can result your amp to be "fried". Old Nelson Pass designs are famous to blow all outputs at once! (very costly repair bill).

Find a pair of G.A.S. products called "The Bridge" (I'm showing my age). This product allowed two channel amps to be bridged into mono. The bad news: Probably hard to find. The good news: If you can find 'em, they should be cheap.

You have several alternatives. First, you could get more efficient speakers. A 3dB increase in sensitivity is equivalent to a doubling in power amp output. Second, I like your suggestion of getting another amp, but rather than convert to mono, I suggest going with a passive biamp setup. Third, you could find someone willing to rip the amp apart and then put it back together in better sonic shape than we you started. Not impossible, but certainly not probable and possibly more costly than you think. Finally, you could dump the Nakamichi and purchase a newer, higher powered Pass designed product. It will sound better, but it is the most costly alternative. My suggestion is to try #2 and if you're still not satisfied, go to #4. At the same time, give #1 some thought.