Running at 15 ips, 2 trk allows full spectrum recording at high levels. Recording at 7. 1/2 ips reduces your HF dynamic range by almost 10 db, depending on what tape you use and changing from 1/2 trk to 1/4 trk can cause you to loose another 6db of dynamic range. Revoxs are a very special case in that the HF frequency response doesn't demolish that much when changing from 15 to 7 1/2 and even 3 3/4 can be acceptable, but only with High output low noise tape. I prefer 7 1/2 2 trk with my B-77 to reduce tape drop output, when I recorded music back in the late 70's, but I used professional DBX noise reduction to increase signal to noise and reduce modulation and asperity noises. Before DBX I used my Professional Ampexs with AME Equalization to pick up an additional 10 db of signal to noise above 2.5 Khz or so running Scotch 203 or 201 , but when Maxell introduced UD tape another magnitude of performance was gained over the Scotch. Ampex heads have wider gaps so the frequency response drops off sooner than a Revox, but the Ampex machines can put more level on the tape and take it off with lower distortion and increased dynamic range. During that period of time before digital Signal to noise was the most important specification. The Japanese thought it was reaching that area above 16 kHz as the most important spec. I'll take a quiet recording every time over one with a few extra HF notes..
Back in the 60's Everyone was amazed when Ampex developed a machine that had 70 db signal to noise below 1 % distortion. That gave us 76 db signal to noise by older NAB standards and close to 86 usable dynamic range. By the 80.s with the new high output tapes we picked another 4 to 6 db of dynamic range. So if you could make the final mix correctly before putting it on the tape and then used that tape to drive the record cutting machine you could almost equal Direct to disc recordings. I said almost. Digital recordings meant you could make multi channel recordings , mix them down later, add effects, and make multiple tapes to drive the recording lathes without introducing any noise, something analog couldn't do. But if you have ever heard an analog tape made with Professional Dolby or DBX, you'll scratch your head and ask your self what's the big deal with digital. There was a quality below 1000 HZ that digital never really captured in my opinion. Yes it was a coloring, but its what you heard at a live un-amplified concert. Something that's missing today most of the time.