Max. volume from GK MB150

Use one that is 8-ohm impedance and that's efficient (high sensitivity, like >100dB).

I used an early GK Micro 150E head with two Eden D210t cabinets for years in a LOUD blues trio (guitarist who toured briefly w/the Nighthawk and w/Debbie Davies, and the original drummer from the Robert Cray Band) and never had a problem with keeping up with those guys w/o p.a. support.

The only issue I had with that amp is it would sometimes shut-down momentarily when it got hot running at 4-ohms. I took care of that by using a small clip-on fan, blowing at the side of it where the vent is and across the cooling fins, to help keep it cool.

 

If it's the 1x12 combo, a matching 112 is the best match every time.

 

In this case, the MB150S is a sealed 12" with a foam surround. It's very rare to see this because it's more of a HiFi design feature than pro-sound.

If getting the maximum volume is the priority, then don't use the built-in driver.
Just use an external cabinet. The 150w is plenty to get good volume out of a decent cabinet. But you could also pick up a new GK combo with a lot more volume for a lot less $$$ than you can a matching cabinet for the MB150S-112III

 

I've seen a local guy here use a Bag End S-15 as an ext. (maybe alone w/ the 12" unplugged?) and it sounded killer. But this was MOR with amplified acoustic, drum machine, etc. I doubt if it would keep up with a loud rock drummer.

 

Like seamonkey suggested, turn off the switch for the internal speaker and connect either a bigger (2x10? 2x12? 4x10? Etc?) 4-ohm cabinet or even two bigger 8-ohm cabinets.

I did that once in a while with my two Dr. Bass 1x12's and got some decent volume out of the amp that way. I'd guess that with a pair of larger 8-ohm cabinets, especially if they were pretty efficient, you could probably move enough air to get some surprising volume out of it.

 

I also have one and bought a "like new" SB112 Avatar 8 ohm to help it out and it seems to be a good match.

I can never figure out why these amps are so expensive for no more power than they have. I also have the MB200 with two GK 12's and it's all I need for what I do. Why do they cost so much compared to the other GK combo's??