I performed a bit of "brow-beating" on MF, and ordered the SL600 and a 4003 FireGlo... They price-matched really well!! Not quite as low as YOMS for the SL600 because I had no official "proof" other than this forum (which they wouldn't take)... But the shipped cost was only $25 more!
Would it be worth getting the SL900 and comparing them side by side? I never did find a great answer about the difference between the 2 power levels... I've bee running the RH450 at 8 ohms through a 1x15 (and sometimes a 1x12), which is about 270 watts... I've never had the master vol past 1 o'clock! Of course, I'm not playing arenas either... So, I'm guessing and hoping the SL600 will be a good match volume wise.