#1
For gigging and band practice. Looking for something fairly mid-range that leans towards a somewhat punchier sound. Band is somewhere within the alternative/ experimental/ atmospheric kinda sound. Will probably buy used to save money, so price range is a bit more flexible but not trying to spend more than 600, and would happily spend less.
#2
First, a few questions...

Where are you located?

Why are you starting with a Bassman head?

That head is overrated (literally) -- Fender claims it at 135W, but it's probably closer to 100W. They did that back when because other manufacturers were over-rating their heads to a similar degree, even with the same components. So 135W is more marketing hype (even for those with the ultralinear transformers) than usable power.

There aren't a lot of people who would use that for a bass amp (more use it as a guitar amp) unless you're doing some relatively quiet things. But you're looking for "punchier."

Frankly, I'd probably start somewhere else than with that Bassman head.

For any bass head up to 700W, my favorite speaker cabinet would probably be a fEARful 12/6/1 or the equivalent. You can actually get that in a DIY kit and put it together yourself (drivers and crossover components are included) from SpeakerHardware.com, thus saving yourself some bucks. http://www.speakerhardware.com/fearful-1261-flat-pack-driver-kit.php

There are very complete instructions, but there's also a video that shows generally how it goes together (you'll want some bigass furniture clamps and a good square), but they're only dry-fitting it in the video. You'll use PL Premium glue and some brads or screws in the actual assembly. Finishing it off might require you to crank up your round-over bit in your router, adding a paint job using Acrytech Duratex (cheap, easy and essentially what the pro audio folks are using) and adding a grill or grill cloth (also available from Speakerhardware.com).




http://greenboy.us/fEARful/

The result is a small, light cabinet (you can add corners, feet, handles, etc. separately) that is absolutely awesome in sound quality. This is the 12" LF version. There's also a version that uses a 15" LF unit (same basic driver), and that's bigger (about 20" x 30" x 16" deep) and louder, but not much more expensive, and the sound quality is identical. The fEARful 15/6/1 will handle up to 900W without straining. Also available from the same source as a DIY flat pack kit with drivers, glue, instructions, etc.

You should be able to Google and YouTube for further information.
#3
Well I wanted an amp that could handle both guitar and bass because I play both in a couple different bands and money is sort of tight. I was also looking for something already assembled so if you have any suggestions on that it'd be cool.
#4
You still didn't tell me where on the planet you're located.
If in Europe, you might check out Barefacedbass.com -- Alex Claber does a great job of speaker/cabinet design to produce lightweight cabs.

Here's the thing. A "normal" bass cabinet will usually color your guitar sound one way or another. And you'd probably launch the voice coil of a standard guitar speaker across the room playing bass through a guitar cabinet.

I've got these same fEARless/fEARful cabinets and because they're pretty flat response, I use them for keyboards, bass and modeled guitar (guitar through something like a Pod XT and then through a power amp).