Re: Guitar cab for bass?
As others have said the guitar speakers will be fine if You back off when they start to complain. Resist the urge to jam with others though, that will probably spell disaster.
So the Hartke is a 4*10" combo?
Hartke replacement speakers are quite cheap at least over here, but I'd recommend upgrading to Eminence Deltalites, which are a good substitute that will most probably work without changing the porting of the cab. Since Eminence gives the full T/S specs on their web site, and also cab and porting suggestions on their tech pages, the upgrade is a breeze. After that, the rig will serve You well for many years.