I dunno.. I have a super old block letter 5150 and it sounds noticeably different from my 6505. Ive swapped tubes between the two and they still sound different. The 5150 is a lot more saturated in the low end and sounds bigger and the 6505 is tighter and seems to react faster. Their could very well be something slightly wrong with my 5150 (its super old and missing most of its knobs and been on a million shitty tours) but it still sounds good.. just different from my 6505.