The 337 is a really nice amp. Very fun, very smooth, and a great soundstage. Not all that different from the A18 actually. I believe I prefer the A18 but I'd have to hear them back to back since it's been a while. Also, I never had the HE500 back when I had the 337 (HE500 didn't yet exist at the time) so I really don't know how it pairs. If you like it a lot, stick with it, if not, the A18 is a worthy choice.
thanks for ur reply...and yeah.i love the 337 especially with the tubes i have in it with my he500..but was just curious if the a18 was a big upgrade..