I have one of these combos. Open back, one 12" speaker. It's a guitar amp with reverb unit. I use it for my fretless bass, and was wondering about converting it from open back to closed box or ported. Possibly changing the speaker unit. Interior volume would be about 26 litres. I haven't had any problems putting a bass through it and don't play too loud - use it for rehearsal jazz band and gigs in smaller venues. May be a case of "leave well alone......." What are your thoughts on this?