Congratulations on getting into the RTR sound with TTP and a Studer A810. Both are quality products and should more than rival the same music on vinyl. However, just as with vinyl, you need to take care of the media and the machine.
Most of the tape available on ebay was produced when 7 1/2 & 3 3/4 ips and four tracks on 1/4 inch were an accepted media. You might be lucky enough to find an exception but the quest might not be worth the trouble.
There are those out there who own master tapes, or more likely copies, that will sell a dub, but you buy them without hearing them and the cost is higher per reel, than TTP. In most cases the sound is still better than the vinyl recording but you take your chances.
Regarding the operating position of the A810. I own a pair of A810s and originally ran them on their back. They ran extremely hot. One got so hot that the pinch roller softened enough to stick to the tape as it passed the capstan. It doubled around the pinch roller and ate about 15 inches of my TTP issue 2 tape. Fortunately, it was at a spot where a loss of one second of music wasn't noticeable. At the time I was making a safety copy of that tape! As it turned out, both machines had the original pinch rollers. Studer outsourced some of the rollers and determined that the material their supplier used wasn't cured properly. I purchased new rollers for both machines to eliminate the problem from reoccurring. I also stopped running the machines in a horizontal position. Stand them up and they'll run cooler. You can also get better cooling by loosening the two screws at the bottom left and right corners of the pres. This will enable you to swing the bottom of the electronics out which makes it easier to operate and let a bit more air in at the bottom of the machine. This feature was designed into the machine to make it easier to operate in tight conditions.
You also need to demagnetize and clean the tape heads every 8 hours or so depending on the speed you run the machine. Not doing so will result in a loss of highs you may attribute to poor tape or machine performance.
You'll need to grow into the do's and don't of RTR machines and tape media just the same as you did while wringing every bit of sound out of your vinyl.
good luck, Ken