I have a SX780 (next up in the line) they are very nice stereo receivers. I use it to power a pair of Klipsch ceiling speakers in my kitchen/dining room. Here's some info on the SX 750;
" The SX-750 was introduced in 1976 and manufactured through 1977. As with other new SX series receivers introduced in 1976, it featured a silver analog AM/FM tuning dial. The SX-750 was rated at 50 watts per channel and had performance specifications that all but matched the top-of-the-line SX-1250 except for per channel output. List price in 1976 was $400.00. The SX-750 and more expensive models in the receiver line were manufactured in Japan. The 'Silver Era" Pioneers remain collectible receivers today."
It looks like SX3700 is a newer vintage without analog tuning and "may" be less collectible.
Here's a link to 'The Silver Pioneer Reference Site' for more info;