Actually, I have one of those cabs sitting in my drummer's rehearsal room....it's been there for 7-8 years easy, and I gigged it for awhile before that. Really not a bad cab at all for what it is.
It seems to me that if you replace the 150x-4 with a 150x-8 you will change the overall impedance of the cab. With a higher impedance 15 you may also find that the 2 8's are getting more than the normal amount of the power due to the higher impedance 15, but admittedly I'm not certain about that part, as I don't recall what sort of crossover setup is in there.
Curiously, the 15 let loose on mine a year or 2 ago...I just replaced it w/ stock, which I happened to have on the shelf. (Don't ask, I'm a hoarder of bass and PA gear). I may have another 15 for your cab in the garage...let me know if you end up needing one and I'll take a look.