Can you hear the difference in R2R format

yes, night and day difference!

depending on the tape and the music recorded, yes you can hear it. It is quite obvious on my system.

If you play a 7 1/2 ips at 3 3/4, you'll definitely hear a difference. Bada bing!

Okay, now that I've gotten that out of the way for all who follow, if high fidelity is your quest I wouldn't even bother with 3 3/4 ips tapes. Which causes me no end of head-shaking when I see the prices that some 'pop' commercial tapes go for. I can only assume that the buyers are discography completists, uninterested in actually playing the tapes. Or at least, that's what I'm hoping.

I had a tape deck that had much better 3-3/4 ips performance than it typical. It was a Tandberg, and it was linear out to 20,400 Hz. I often used 3-3/4 ips to record albums, but this was in the '70s, and I didn't extract nearly the dynamic range from LPs that I do now.

However, if I was dubbing from a truly high end recorded source, and that includes the few classical Advent Cr02 cassettes produced in the '70s (or direct-to-disc LPs), I needed 7-1/2 ips. And if I was recording live, nothing less than 7-1/2 ips would do, and even then I tended to use Tandberg's peak limiter.

Furthermore, true usability at 3-3/4 ips also depends on the quality of tape. It took a top end premium tape from TDK, Maxell, or 3M to be able to use 3-3/4 ips without hearing dropouts from tape oxide flaws.

Tape age and condition can be a major factor today, adding to the difference in the quality of the sound. From buying 'lots' of commerical recorded tape, I sometimes get duplicates and often when I play each, different quality of sound, that goes for three's and seven's. So, condition is a big issue today. I don't do any recording, just playback. Since I have more then one deck, playback is also affected by the alignment of the heads. Still, some of my tapes sound better then the vinyl and cd, you figure.

As a general rule, I won't buy any reels on Ebay that are recorded at the slow speed. That said, I do have a few slow-speed tapes which don't sound half bad. It's pretty much a crapshoot in any event, since all the tapes available today (with the exception of Tape Project reels and a couple of others) are at least 25 years old and heaven knows how they were treated.