We have been experimenting with a new modification to the RS1500 tape path that is really nice. The fixed guides above the pinch rollers - which are made of stainless steel and are often worn flat - have been replaced with ABEC 5 ball bearings and ceramic edge guides.
(http://www.tapeproject.com/machines/RS1500-roller-guides.jpg)
If you look at an ATR 100 series machine you will guess the heritage of this idea. After a fair amount of time spent looking for the proper components I have come up with a combination of bearings, precision bearing shims and ceramic edge guides that perform beautifully. Along with the great sound one of the most striking things is how quiet the machine gets with two more points of friction for the moving tape having been eliminated.
The parts for the mod are not cheap, but they are also not terribly expensive. So I have decided to make this an integral part of the tape path mod. The price for the tape path mod is now $600 and includes this upgrade.