I run an Ampeg Micro VR 200 watt bass head through an old Fender Bassman 15" speaker cab (rated at 150 watts, the Ampeg runs at 150 watts as well on 4 ohms) but it's just not quite loud enough or tough enough... the speaker tends to distort a bit on certain notes when the Ampeg head is running over half volume. I'm assuming this is because the speaker should normally be rated higher wattage than the head to allow some headroom.

For $180 I can get a Celestion BL15 400X 15" speaker rated at 400 watts 8 ohm, so I'll be getting maximum output from the Ampeg and still a lot of headroom so won't blow the speaker (which I'm worried about with my current setup).

I've read that you can't just bung any old speaker in any old bass cab... would this Celestion speaker work well in my Fender Bassman 15" cab? Really don't want to spend heaps of money on it, just need to get maximum out of the head unit without risk of anything blowing up at gigs and figured the Celestion was a relatively cheap upgrade...
1. The old Bassman cabinets weren't "designed" around a particular speaker; they're more like guitar cabinets, for which a guess was sufficient.

2. Bass speakers are less about matching wattage than about cone excursion limits. These can be affected a lot more by cabinet design than guitar speakers.

3. Might be that your head is clipping rather than that the speaker is distorting (or it could be a combination of both). Remember that total wattage sent to a speaker has nothing to do with where the volume knob is set. Guitar players think that if they're getting insane volume out of "1" on the volume knob that they've got huge quantities of wattage in reserve. Not true; you may actually be using ALL of the wattage of your amplifier at that point. Bass players realize that it takes much more power (and at least four times the air movement) to play a note an octave down at the same SPL as the original note.

Bass amp heads can clip very quickly, particularly low-wattage heads like the Ampeg Micro VR 200 you have. That head is designed for LOW volume work. Realize, too, that the VR 200, while rated at 150-200W at 4 ohms, is likely to be putting out half that power at 8 ohms.
Ah right... Yeah I assumed this particular cab was part of their bass guitar specific cab range, being a 15" speaker. They also do new versions of the exact same thing that are specifically for bass amps, which are three times the wattage.

I just have no idea about bass speakers and cabs and how to know what will work in what cab... so I posted the question to see if this would work.

I know the Micro VR head is meant to be a practice amp but for how I use it it's just loud enough to hear on stage, and with a DI on it it seems to work okay. It's rated at 150watts 4 ohm but goes up to 200 or 220 watts at 8 ohms so that's why I was thinking if I got an 8 ohm speaker rated at 400 watts I might get more out of the head without risking the speaker blowing out (as I was told by Fender that running a bass head full bore through a cab that's only rated the same wattage as the head can damage the speakers)
The VR produces 150w @8 ohms so if you want more loud you need a highly efficient 15 rated at least 150w @8 ohm. When in doubt, RTFM

The sensitivity of that Celestion is only 96db so if you want more loud, look for a good used Altec/EV, JBL, Eminence with a sensitivity at least 100db. Much more louder and probably better sound.

"Your sound is in your hands as much as anything. It's the way you pick, and the way you hold the guitar, more than it is the amp or the guitar you use." -- Stevie Ray Vaughan

"Anybody can play. The note is only 20 percent. The attitude of the motherfucker who plays it is 80 percent." -- Miles Davis

Guthrie on tone: https://www.youtube.com/watch?v=zmohdG9lLqY
Balls... sorry I had it the wrong way around. My speaker is 8 ohms, so runs the head at 150 watts! So I should've said I was after a 4 ohm speaker of at least 200 watts in the original post...

What about the build of the cabinet enclosure? I read on another forum that for bass cabs you need to make sure the speaker is right for the cabinet and vice versa...