I beleive the 710 and higher were praised for their dome mid, as you are attesting to.
If they are the ones with the metal grills and are in as nice a condition as you say, $400 would probably be the low end. A pair of L1510's have been lagging on eBay for a while, unfortunately for that seller the 710 and 810 seem to be the more sought after version of that A/D/S series.
The woofers were tragically underdone. Examination of the driver will be reminiscent of the Dynaco A-25's bass driver with a narrower surround. Sad little magnet structure, but true-sounding with little or no distortion. Try adding something along the lines of a Dahlquist sub with an outboard crossover and dedicated amp.
I have heard a pair of 710's running off vintage Pioneer seperates, I think it was an 8100 integrated amp. The sound was extremely clean, and I will echo your perception of a weak low-end but vast soundstage. It's all in the mid.