To me, this is a simple matter of trust.
I have no reason whatsoever to not trust the Audio Research technical expert's statement. Also, since my REF 250s are probably out of warranty, I really don't want to take this chance.
But, people being how they are want to upgrade and experiment. In cases like this, a simple phone call to the manufacturer is always the first thing I would do. no reason not to trust them.
George Meyer audio is an authorized repair service for several brands, including Mark Levinson and Audio Research. They also told me that dropping KT150s into the non-REF series is not a good idea.
Let me just say this. no company would replace/upgrade the power supply transformers just to sell an SE version if it wasn't necessary. I haven't seen one example yet of Audio Research telling people not to tube roll for other amps. Only in the case of the KT150s have I seen this recommendation.
So, as the saying goes, you takes your chances. When the transformer or other components fail, don't stand their wild eyed and say, wow! I didn't expect that.
This is like taking a stock turbo charger out of a car and replacing it with a monster turbo without making the necessary adjustments to timing and intake, etc. to ensure the motor doesn't blow up with too much boost and internal pressure.
It'll work for a while, but when it goes, it will be spectacular and dramatic.
I know the OP decided against this, but for me, when I can afford to upgrade to SE and I deem it necessary (too totally different things), I will do so.
What I would like to do, is (asking for trouble now) borrow a pair of REF 250 SE amps and do a side by side comparison with my REF 250 amps and actually hear the difference. That would be fun.
Long post, sorry.
I heard a dramatic side by side difference between my REF 3 pre amp (still an outstanding pre-amp) and a REF 5SE pre amp. Wow! even my non audiophile guest heard and appreciated the sonic differences. I did not hear that much of a difference between the REF 5SE and the REF 6. However, truth be told, the REF 6 came to me with only 24 hours on it. So, since I traded the REF 5SE in for the REF 6, I couldn't hear the side by side after the REF 6 was broken in.
As with high performance cars, same with electronics, if one knows what they are getting into and can afford the consequences, then as long as one's eyes are open, do what you like. But, if you can't afford to replace thousands of dollars of blown transformers and other components, I definitely wouldn't go down this path.
enjoy
