I've had both. ADS owned include: L1290s series 1 (1983-84 vintage), M15s (different than M1590, but pretty similar), CM-7 compact monitors, contemporary of the M-series.
M are the later series, after Mike Kelly was out of the picture. Ultimately, I preferred my L-series over the much more expensive (at least for me at the time, both pairs were used) M15s. The M15s, despite having 2 10" woofers to the 2 8" L1290s, had less bass! significantly less. The top end was pretty good on both, but again, I think the nod goes to the Lseries. L-series come up on ebay from time to time, along with a few spare parts on occasion. the L1290s and 1590s could be biamped with active PA-2 amplifiers that fit into the rear of the speaker. Supposedly great sounding, though I never had a set. THey are rare but do trade hands on occasion. The M-series use poly tweets, woofs, and mids (dome or cone) where the L series used paper woofers, and the 'sticky' dome tweets and mids. If the price is right, the M1590s are still a nice speaker, but I'd take L1290s or 1590s over them. I'd say $1000 or less would be a fair price for M1590s, I sold my M15s a year or more ago for that amount.
Have fun,
-Ed
M are the later series, after Mike Kelly was out of the picture. Ultimately, I preferred my L-series over the much more expensive (at least for me at the time, both pairs were used) M15s. The M15s, despite having 2 10" woofers to the 2 8" L1290s, had less bass! significantly less. The top end was pretty good on both, but again, I think the nod goes to the Lseries. L-series come up on ebay from time to time, along with a few spare parts on occasion. the L1290s and 1590s could be biamped with active PA-2 amplifiers that fit into the rear of the speaker. Supposedly great sounding, though I never had a set. THey are rare but do trade hands on occasion. The M-series use poly tweets, woofs, and mids (dome or cone) where the L series used paper woofers, and the 'sticky' dome tweets and mids. If the price is right, the M1590s are still a nice speaker, but I'd take L1290s or 1590s over them. I'd say $1000 or less would be a fair price for M1590s, I sold my M15s a year or more ago for that amount.
Have fun,
-Ed