It sometimes pays to mess around with your gear!

I've had this older Behringer BX600 bass combo amp for a while now, and while I was always more than happy with the amount of loud this 60W amp produces, I was never, ever happy with the tone. It was always much too trebly, and had almost no bottom in it. It has the 'upgrade' speaker in it that handles 100W, but Id almost bet it's just a relabeled guitar speaker, because of the frequency response of it. For bass, it just plain sucked. I also did a little 'reverse engineering' on the two ports in the cabinet, anf found that the enclosure is tuned at approx. 65 Hz. Not good, for even 4-string basses go down to 42 Hz! Plugging one port up with a dense foam lowered the tuning to a better 45 Hz, but the speaker still didn't want to really go down there at all.
I started looking for a 12" hi-fi woofer to put in it, figuring that would tell me if the speaker, or the amp itself, was the culprit in the bad, no-bottom tone of the combo. So I found an inexpensive woofer at Parts Express, very cheap (I won't even say). Its specs said that it was good for 120W (twice the output of this amp), and flat from 35-5KHz. I figured that was enough highs, since even Eminence bass drivers are flat only up to about 4-4.5K.
Well, I figured right on the upper frequency response, as this speaker puts out plenty of it...and it's a woofer, not a 'full-range' instrument driver! It also improved the low-end 30 zillion percent. I tried it with both ports open in the cabinet, and found that while the lows were 'there', that they were rather 'floppy' sounding. So I tried it with one port plugged. Better, but still loose and flabby in the lower ranges. Then after remembering that the description of this woofer on the Parts Express website stated that it was designed for sealed enclosures, I put the plugs in both ports. Voila! Solid, tight lows! Now, this driver's efficiency is not as good as the usual musical instrument driver, at 89 db as opposed to 95 or so, so the overall volume output of the amp suffers a little. But it's basically my home practice amp anyway, and thus still much louder than it needs to be for that use. But it's still loud enough for an intimate coffeehouse acoustic or jazz gig, and for general garage-noodling. Time will tell if this inexpensive woofer with its foam cone surround is going to hold up as a bass amp driver, but for the price I paid for it, even if it lasts only a year, it was money very well spent.
So, if you're anything of a DIY musician, and a piece of your gear "just ain't right", don't be afraid to experiment on it a little. This little experiment of mine brought my little bass amp from being something I wanted to trade away on Craigslist, to something I can actually use and enjoy now. If you have a little bass practice amp, for instance, with an 8" driver that just doesn't put out good bottom, try a hi-fi woofer in it, but go for one that's rated for about twice your amp's output. Chances are, it will have enough highs for you, even though it's a 'woofer'. And chances are very good that you'll see a rather large improvement in the low end.
Go For It!