I was feeling exceptionally muscular the other night and decided to take the Twin Reverb to rehearsal. I don't know the exact year but it is mid 70's with JBL D120's in it. Push/Pull MV, not the ultra-linear. Everything sounded as clean and bright as ever, however the low-end sounded kind of "farty". It was re-tubed a couple years ago, the caps have never been changed. Any ideas what to look for before I take it to a tech? Thanks!