My practice room real estate is at a premium with no room for may main rig. I have a small, inexpensive low wattage guitar amp in there hiding under a table. I mainly play guitar but sit in with some guys that need a bass so I fill in as best I can. I plug into this little amp and it sounds like you would expect, like crap. I don't care really, I just need to hear myself when I'm noodling around or working something out. No need to rattle windows. The little 8" speaker breaks up pretty easily at moderate gain. Question, if I put a real 8" bass speaker in there, would I get much improvement? Thanks.
No, even with a bass spkr, it's a guitar cab, and most likely open-backed. Best bet would be to buy a small bass combo, like Ampeg BA-108, for very little $$ but then your bass will sound like a bass.
+1 I take mine on family/business trips all the time. It has mp3 player inputs and a headphone jack for silent practice also and it really does achieve the Ampeg tone, though without much air movement.
A small bass amp (ie: at least 90 watts with at least a 12 inch speaker) will give you the best sound and durability. My nephew had a Behringer Ultrabass 900 (http://www.behringer.com/EN/Products/BXL900A.aspx) which was good for practicing if the drummer had e-drums and turned down a bit. Or spend a bit more (or quite a bit more) and get the tone you want in the size you want at the volume you want with Markbass, Phil Jones, or one of the other "loud AND small" combos. The benefit there is you can usually add on a second cab for full gig worthy!
I wouldn't wast your money trying to turn a crappy guitar amp into a crappy bass amp. I use a Squire 10 watt guitar amp with a 6 inch speaker (if that) and it works great if you ask me. It came with a cheap guitar I bought for my kid years ago. The sound is half decent and with no bass to speak of you can hear every clink and clank which helps smooth out your playing. I have a couple of other amps for gigging, but for practicing at home I use this little thing all the time. Granted if I used it with my friends I'd be SOL.