Interesting comment on SS vs. tube power... I would have guessed the opposite, with about 2x the power in a tube amp to match the bass tightness of a SS amp. In the midrange and up, I expect power to be a wash, with the tube amp sounding a little "sweeter". I have always felt bass to be one of the weaknesses of tube amps, primarily due to their relatively high output impedance (and low damping factor). BTW, assuming a good amp with stiff rails, the SS amp rated at 600 W should put out 800 W into 6 ohms; the tube will put out 300 W no matter the load as long as the tap is there and it is matched to the speaker's impedance.
88 dB/1 W/1 m, say we want it to reach 120 dB peaks at our listening spot, maybe 3 m away. That means we need an extra (120 - 88) = 32 dB, and another 6 - 12 dB for distance depending upon the room gain (size etc.) Let's say an even 40 dB, or 10,000 W (!) Maybe 110 dB is more reasonable, yielding 1000 W -- closer to your target goal. In fact, 600 W is about 28 dB above 1 W, so the speaker should output 116 dB at 1 m and maybe 110 dB at the 3 m listening spot driven by 600 W with a little help from the room; close enough! Good "guess", Gregadd! Leaves the tube amp lagging a bit, though, as it will be 3 dB into clipping at that power. We probably won't notice as it will only happen on the loudest peaks.
I thought this was about amps; it's a "guess the speaker" game? Alas, I cannot play, as I am unfamiliar with most of the newer speaker brands. 6 ohms rules out Magnepan, however.
Onwards - Don
p.s. I am an "objectivist" who has owned tubes until very recently, and sold out to SS due to cost and other considerations. I always did like falling outside the box...