I have the same 637/627 M^3/Sigma and GS-1 as you do. Unless the 637s are so much better than the 627s
I can't agree with your evaluation of the M^3 as bass heavy, bloated and slow. I won't argue that the GS-1 has greater resolution. But it's just as easy to say that the GS-1's wire with gain sound is bass light, bright, analytical, and lacking the organic 'body' the M^3 has?
Did you build your M^3 or get it new? If not, how old is it and how many owners did it have? The reason I ask is that
not all M^3s sound the same. I have 2. The older one (8065) was set up to swing as much current as possible for low impedance headphones like the AKG 340s and has a brighter sound signature than the newer one (637/627) which sounds remarkably like my Cetron tubed Woo3.... and if you know Woo amps, they don't sound tuby, bassy, bloated or slow, but more like good solid states.
Now
what headphones are you using to make this statement? I see you and I have had 650s, 701s and 880s in common. If I were to use each one of those headphones to describe my M^3, I would come up with 3 different descriptions. There's no way I could call either M^3 bass heavy, bloated and slow with the 701s. I find both M^3s lean toward the brighter side of neutral with the 880s, and the 650s have the warmest, but most accurate tone with either my M^3 or my GS-1.....
Now I see that you have
transitioned to expensive ear buds. I had the UM3X but sold them. If you had to say which of your
universals was the best, which would you pick?
Yes you do.
USG