pm1200 losing bass output

I have a pm 1200 and a pm 900 from around the early/mid nineties. Only used at home now. I am running one set of speakers and have
just noticed if I switch from the 900 to the 1200 all the bass dissapears? If Ipull down one of the channels all the bass comes back.?
So one input on its own gives great bass response, as you push up the faders from the desk the other speaker comes on but all bass nearly gone. Tried all combination of leads
speakers etc but as I said with the 900, same set up full sound. I am no electronics person but just need to know so i can take a decision whether to get it fixed.

Sounds like gou have the speaker wiring out of phase.

Thanks for response. I thought about this but if I swap to the PM900, same cables, leads etc, works fine??
Second speaker comes online, bass stays in the sound and the whole thing gets louder. ???

Has it always been like this, or is this a new development? If it's always been like that, I'd take a look inside the speaker & see if anything is wired wrong. I've had TWO pair of Klipsch speakers come from the factory with speaker leads on the wrong post~

thanks for response.

Can't be speaker as with other amp sounds great? Used to gig these regularly, no problems. Sat in house for about 12 years but has recently been used for band to practice with so I'm wondering if something has gone inside? gomma take it to original supplier and get it wired up to there desk/speakers and give it a listen.

Sounds like the amp needs service or replacement. You could probably find one working for relatively cheap on eBay but may run into problems down the road as these amps are nearing "vintage" status.