I've run bass through my 18 Watt normal channel with a closed back 2x12. Sounded good, just that the audible volume is pretty low. It would be fine for quiet practise. Of course you don't turn the amp up much so it doesn't go into much if any distortion. Also if your bass pickups are high output they could overdrive the first stage. If that's happening and you want clean just turn the bass down a bit.
crgfrench was proposing that we first turn the amp into a 36W, which is an idea I agree with, in addition to the 2X12 closed back cab. Should make for a pretty decent bass practice amp, with sufficient clean headroom.
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