Need some audiogon expertise

Have you had this system long? The components have been around since about 1990. Did the system ever provide a fuller bass? You're right that it should sound good. There's enough power and the speakers are designed to provide you with solid bass. I'm wondering if the thin bass is a recent phenomenon? If it is, then here are my questions:
1) Did you change anything lately (e.g., cables or room)?
2) Is the problem present in both channels?
3) Has this been a gradual decline in bass response or sudden?

Great points made above. Speaker positioning is generally a good starting point. Your room dimensions aren't bad so that shouldn't cause too many problems. I assume you have a fairly standard room -- i.e., not too many large openings into other areas, not too many hard surfaces, a decent amount of carpeting and cushioney (is that a word?) furniture. If not, then start with the room itself, especially if it seems like the speakers are throwing out decent bass if you put your ear (or meter) right next to the woofer.

I also assume that the lean bass comes across whether you use your tuner or cd player so I wouldn't look for answers there, at least not right away. If it's easy to borrow a decent preamp, you might want to switch that out to see if it makes a difference (by the way, as Sean suggests, it may be the amp/preamp cable rather than the preamp itself if the borrowed preamp sounds better -- i.e., impedence/inductance/capacitance matching may be off). Finally, you might want to borrow other speakers and see if that makes a difference. If so, then you may have some worn parts. Visually inspect the woofer cones for tears or cracks -- if they are there, replace the cones. If everything still looks good, consider replacing the capacitors in the cross-overs as a final last resort once you determined it is the speakers.