My amp has KT-88/98 output tubes (4), 12AX7 (2) tubes that can be called "input" tubes, and four 6SN7 tubes. The amp sounds a bit bright and is missing some midrange and some tightness in low tones. I hear about rolling output tubes, and I believe rolling the 12AX7's would also impact sound. Would rolling the 6SN7's impact the sound also? This amp can also accommodate 6N8P or 6H8C tubes in place of the 6SN7's. How do these other types compare with the 6SN7's?