Now it's subs

" If adding a sub to a 2-ch audiophile system - i.e., not a home theater receiver withy built-in gizmos - don't you need an external crossover, to prevent your main speakers from getting the bass signal?"

It depends. In many cases you don't need, or want to remove the bass from the signal with an xover. You adjust the sub so that it fills in where your main speakers naturally roll off. 

Another option is to adjust bass frequencies with software if you have a computer audio system.

" Today I was just testing a recovered old active Gallo sub and it made some just okay speakers sing."

Sometimes, when you listen to a system that you're not used to, you can have an initial impression (good or bad), that may not hold up over time. Its always a good idea to confirm results like you got with some long term listening. I know that you didn't hear the sub in your system, but the same rules apply.

As some of the others have stated, your best bet would be a pair of Vandersteen subs. The only caution I would give is that subs usually work best with full range speakers. Your speakers are fairly small, so you can't expect the sub to handle frequencies that your main speakers should be covering. (That goes for any sub, not just Vandersteen.). I do have a pair myself I use with Model 3's.

jafant +1

RELs take the signal from the amp and allow you to leave the mains alone, or you can unplug the mains and listen to the sub by itself which doesn't sound very good at all. You can also leave the amp off and hear nothing from either the mains or the sub, or you can turn the whole damn thing way up, pack up the car, and drive to Mexico.

RELs are unique in that they can take the high-level signal from an amplifier to better assimilate with the signal to the mains as you say wolf.

I am using my two REL Stadium IIIs in "out-of-phase" mode to cancel some undesirable standing bass waves in my room. Faster and more tuneful bass than without. They still pressurize the room and give the soundstage extra magic.