Freo-1,
I've done some pretty recent 'testing' of alternates for the 6U8..... including the 6GH8, the 6BL8, and the 6HL8. I've compared these tubes in the Scott.... and then listened again to other amps like my Fisher 500B, 500C and 800. I was a HUGE fan of the simple and available 12AX7.....and for years considered that to be the one and only tube that 'should' be in the front end of an amp. Then I got an ST-70 with the 7199, an SCA-35 also with the 7199....some Mk IIs and Mk IIIs w/ the 6AN8s, and even the ST-35 w/ the 6DJ8. Suddenly my world was opening up. Then along came the Scotts......and I really started to wonder. So....many evening were spent just tube and amp rolling. Honestly....to the point of ad-nauseum! I truly believe there is a time when you lose your original point of reference......and I think that happened to me. I ended up just tube rolling in the Scott alone one evening......and came to a very similar conclusion about the 6U8 that I did with the 12AX7; that being that 'good' tubes can sound good.......and crappy tubes can sound crappy!' I hear some pretty distinct differences between the characteristics of a Mullard or a Telefunkin vs that of cheap Chinese or Asian 12AX7.... and even some US 12AXs.....IN SOME AMPS! In other amps......I honestly couldn't tell ANY difference between these tubes. And, that included using tubes that clearly sounded different when placed in other amps. Likewise, I rolled around a number of tubes in the Scott.......and eventually settled on a pair of old US 6U8s that really sounded great!! I found the 6BL8s to be more consistent.....the HLs to be 'lack-luster'.....and for some reason I just can't get my head around the GH8s. But, one thing I'd report as my findings, perhaps not what others have found...but I did note that the Fisher amp seemed FAR more 'agreeable' with a varying level of tubes than did the Scott. I found my Sherwood to be VERY fussy about tube types.......and sonic differences were much more easily noticed in that amp than in any other I tried. Just fwiw..... Tom