15 foot RCA cables out of TT with no pre amp?

The main issue is not only loss but noise you want to keep the connection to the turntable as short as you can due to these reasons try to arrange a way for that to occur so you do not develop noise or hum.

Any "loss' would be overcome a noise sddition. All you can do is try and see if you can live with it.

Yes, keep the TT output cable as short as possible. It’s carrying a very low level signal, and thus is more prone to noise and other issues.

On the other hand, the phono preamp out is "line level" and fine with a longer cable. I, too, prefer balanced connections if the gear on both ends supports them. If you can use them, you can have cables made up from Mogami W2549 cable and Neutrik connectors by a Pro Audio shop (if you are in the USA, ProAudioLA does great work) for about US$100 a pair or less, depending on length, of course. I found them far superior to $2000 "audiophile" cables.

That seems a bit too long.  Typically 5 feet is considered max, especially if you are using an MC but you are ok with Ortofon Blue since it is an MM cartridge.

The MOST important factor in a phono cable is the shielding and the extra capacitance.  When it is too long, the extra capacitance may cause it to increase the high frequency response (treble) to a point where it may be noticeable.  

Voltage loss is not going to be too much of an issue since there is virtually no current flowing in the wire, since the input impedance of an MM phono amp is about 47k and a few extra ohms from the cable will not make that much of a difference.

However, try it and if it sounds ok, you are done, if not, try and keep it shorter.

I would listen carefully to a very familiar lp with distinct lows, mids, highs. Then make a temporary short cable hookup, listen, despite theories, I doubt you will hear a difference.

let us know if you try this.