Well, after some discussions with my brother, we decided to give it a try. We pulled out the Ruby 6L6 and replaced it with SED 6550 Wow! This thing is much louder and warmer with 6550 tubes than with 6L6! With 6L6 tubes the amp is very cold bias but with the SED 6550 the bias is perfect! Sounds almost as an SVT-II but than cleaner and more punch. here is some proof!