Thanks for the responses - I have now switched over to the EL34s in my Nomad 55 1x12 combo and for the good of others, I would like to share the results of the swap! I popped in a pair of the Mesa STR-147 EL34s (gray rating) which are rebranded Sovteks. Well, I must say that I couldn't be happier with the tone!
With the EL34 tubes, my Nomad's channel 1 (clean) sounds like glass - think vintage Fender. With the 6L6's this channel always sounded nice, but I wouldn't have dared to call it extraordinary. I now feel comfortable describing my clean tone as "extraordinarily" clear and shimmering.
Channel 2 (moderate gain) sounds punchier and more present. The basic tonal character that I liked before the swap is still there, but just more defined without any notable loss in the lower end frequencies. I like it better than before.
Channel 3 (high gain) sounds similar to a Marshall Plexi tone. I officially declared this channel "unusable" when I had the 6L6s in there. The change in this channel is the biggest surprise of all and sounds just searing, yet warm and defined with my Les Paul Standard. WOW!
Please note that after the OT swap, I experimented with different preamp tubes in V1: a Mesa SPAX7 (too dark), GT 12AX7-C (great cleans/undefined gain) and a TAD 12AX7A-C (THE WINNER!). The TAD tube was the winner by far by emphasizing the glassy character of the clean tones and a more focused and compressed tonality in the gain on both channel 1 & 2. The TAD was a perfect compliment to the new power tube switch. I tested all of these combinations with a Les Paul Standard and a ash Tele with hot single coil pickups. In the end, I have sonic sweetness all around!
