Long interconnect cables

I run 25 foot balanced to my front amps, not problem.

The are a variety of factors involved in your question. The single most important question is related to the output impedence of your preamp. In short, the lower the better. Long cables need current from source to drive the signal. The input impedence of your amps are also a factor, and there is a ratio between output Q of preamp to input Q of amp that is ideal. The damping factor of amp can also be a factor. Today, generally, 50 ohms is considered a low output Q. (Personally, I like lower still. There is no substitute for current or horsepower...) You should have no problem driving up to a 10 meter length of single ended (RCA) cable if the output of your preamp is in the 50 ohm range and the input of your amps are in 50 - 125K range. As a final thought, you will also want to keep the capacitance of the cable relatively low.
In this context, I'm driving a 7.5 M S/E cable effortlessly.
Hope this helps.

I run 35 feet of balance XLR's from preamp to amps, no problem.

No issues at all, especially if you run balanced cables (XLR connectors). I have done it that way for over 10 years and I am very pleased with the result. My room is set up the same way as what you are planning to do, front end audio gear in a cabinet along the side wall, speakers along the front wall with amps next to them, and 35 foot long balanced Interconnects used with no problems. Short speaker cables (4 footers.) Go for it I say. Balanced also lowers the noise floor (in my experience) and alleviates the potential for system hum, which is much more likely if you run long single ended I.C.'s with RCA plugs.

Im running 21FT single ended sounds better than balanced,my amp is Rowland 8T with Thoress preamp which is tubes line and phono.