Is it safe at low volumes? I have a Marshall dsl 50 and a 1960a cab. Saving to get a bass amp but that may take some time. Is it safe to use my guitar amp and cab?
If you keep the volume low and don't crank the bass on the eq, it should work fine. I have a little 10 watt guitar amp combo that I use underneath my computer and its no big deal. Played it for years. If you start pushing it though, all bets are off.
Guitar amp and cab is OK, its bass amp and guitar cab that is trouble. The guitar amp won't do the lows and power for breaking cabs.
I played bass through a Marshall guitar stack (JCM 900 100W Dual Reverb Head into 1960A and B cabs with Celestion G12T-75 speakers), at gigging volume (played many gigs with that setup), for years before getting proper bass amp gear, and never damaged anything. Cheers!
Isn't volume as much as bass, bass causes excursion, guitar cabs are meant to go really loud, and middly because that is what guitar is all about. The whole Fletcher Munson thing means that going really quiet encourages you to up the bass, which is where the trouble is. Its a 300w cab isn't it, with a 50w head? Very unlikely to have trouble.