Well the free solution is to have him experiment with his listening position, he’s probably sitting in a null. He could also get improved bass my relocating his speakers.
if you’re going to be spending money, I’d suggest getting some better basier speakers, headphones, or a receiver with a sub output. All easily found used.
To specifically just add a subwoofer to a high end 2 channel system, I would suggest a BBE 382i SW, which is a rack mounted unit, or get a Peachtree Nova 150,300,500 with built in sub out but I think a $50 used receiver, or a $200 flagship one with preamp outputs is the best path to a sub out. As they have great performance, and especially with pre-outs you can use a really fancy 2 channel power amp with it, or his existing two channel amp if he prefers it to the sound of the receiver’s built in amps.
Also make sure the speakers are wired in phase, (red to red, black to black) and that the jumpers in the back of the speakers are present if they have two sets of terminals. That’s an easy way to loose bass.
Ya can’t go wrong with bigger better speakers, but you can easily spend money on extra gizmos that don’t get you very far, including the subs which can sometimes be pretty woosy. In one brochure the sub and the speakers had the same low frequency range. So obviously that sub was not the one to get, as you want your sub to go lower and further than the speakers can go.
