It may interest members, particularly ARC loyalists, to know that i was able to directly compare my ref 150 vs a kt 120 retrofitted ref 610, which on paper, seems like a gross mismatch, 150w vs 750w! Driving B&W 800d's, the ref 150 showed how far ARC has pushed the limits of the state of the art. The ref 150 was faster, quicker and in fact seemed to even go deeper in the bass, while at the same time, sounding sweet, smooth and oh so full. Soundstage size was a dead heat, a big surprise considering that the 610, being monoblocs, should have had a clear edge in this department. If the mighty 610 showed any advantage, it probably was slightly more composed when pushed playing complex musical material, as should be expected with all that massive headroom. Of course, if your speakers are extremely inefficient or your musical taste favors big orchestral works, hard rock or home theater use, then the 610's may better suit your needs. I'll probably be trying the ref 250 to determine if the extra 100w is worth double the asking price of the ref 150. But for now, I'm truly impressed how arcs stereo ref 150, in parameters that matter to me musically, surpassed the mighty ref 610. It also seems that simply retrofitting the older ref amps with kt 120's cannot bring the performance up to the level of the new ref amps whose power supplies, power and output transformers and those new Teflon caps were designed specifically to maximize the performance of the kt 120 tube.