50' speaker or 50' interconnect?

Generally, the answer is 50' interconnects, and balanced interconnects if possible.

However, your results indicate that perhaps only experimentation with both speaker cables and interconnects will provide a satisfactory answer.

Low capacitance interconnects can be purchased or fashioned from Mogami 2534 or Gotham Audio GAC/4-1 microphone cable. I also use Oyaide PA-02 for 30' XLR interconnects with excellent results.

...btw, 50' Mogami 2534 mic cables can be purchased from several retailers including Guitar Center. They can be returned within 30 days for a refund.

If you used cheap 50 foot XLR and did not try a LONG break in you are not getting the results possible.
The minimum quality would be the Mogami (not a copy!!) suggested above. The minimum 'audiophile' 50 ft would run you about $1,000. or so (so I WOULD try the Mogami and use it for at LEAST a month or two)
I use a Kimber Hero 7 meter ($700 plus) XLR (half your distance!)
To get a speaker cable that can go 50 ft is going to cost a LOT more for the same quality.. so stick with the XLR IMO.