The S stands for "Simul-Sync" which enables you to monitor previously recorded tracks while recording new ones. The the 3340 didn't have this I believe, and also only had 7 inch reel capacity, unlike the "S" version, which can accommodate 10.5 inch reels. I owned a 3340S and liked it from a sound quality point of view, I didn't have an issue with noise, but I did record at 15 ips all the time. However, from an operational point of view, I thought the machine to be an absolute nightmare, especially this silly simul-sync system, I could never remember how to set the switches, and it was always really frustrating to use. I would go for something like a 3440, if you want a TEAC, and a good mixer, with flexible routing and monitoring options, rather struggle with this ridiculous switching system on the 3340S. I guess they had to cram in a lot of features on the S version like this monitoring system, to make up for lack of facilities on the TEAC mixer that was made for it.
http/www.angelfire.com/electronic2/vintagetx/TeacA-3340S.html
As for used prices ? who knows, I guess about £150 or thereabouts, I certainly wouldn't pay anymore, if you do your being ripped-off. Vintage or not, this isn't really worth paying a lot of money for, quarter inch tape, limited track count, and the high cost of tape these days, about £40 a reel ! does this make any sense anymore, I can understand if you were going for a Studer multitrack, or an Otari, something that's really worth having, but a 4 track ? why not get something digital ? it's cheap, easy to use, and doesn't cost anything to run, I've been through this analogue thing back in the day, when there wasn't any alternative, when digital came in it was a revelation to us old guys, all of a sudden there was no noise ! and what went in stood a good chance of coming out ! I know there is this nostalgia element, but honestly, I really don't want to be reminded of my days struggling with "antediluvian" objects like this !
Purplepatch