The tubes in my bass head, Marshall JCM 800 bass series, need replacement, but while I'm waiting to get that fixed, I need something to practice on! All I've got is my guitar amp, a Marshall G80R CD, and while I am aware that it will damage or entirely blow your guitar amp's speakers if you play a bass too loud through it - I'm wondering if it would be safe to connect the cabinet that my bass top originally was connected to - to the guitar amp and play it through that?
Would I be able to play loud enough to play along with records, I don't care if the tone isn't perfect and I miss some lows/highs whatever, but as long as the guitar amp stays intact, which it should since all the sound is being processed by the cabinet itself - or is my logic wrong?

Hope for some help, thank you!
Awesome, thanks! By impedance do you mean that the watt/ohm outputs should match eachother?

I just googled the loudspeaker in my cab - it has a nominal impedance of 8 and 15 ohms (don't know why it says 8 and 15, found details here, I have the G12H model: http://www.legendarytones.com/celestiong12m.html), and the loudspeaker in my guitar amp says 8 Ohms, so I guess it's good to go?

Edit: there's two of the G12H loudspeaker in the cab btw, probably doesn't make a difference - but just in case!
You'll be fine as long as you're using a bass cab with matching impedance. Rob Trujillo plays through a Triple Rectifier for instance.
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Greg what did you send me??
I was going to say check the impedance and Lemmy does that too but everybody already said that.