Volume Problem Trouble Shooting Ideas?

I have been trying to figure out the source of a problem and getting nowhere, so I was hoping to get some input from Members here that are FAR MORE advanced than myself. I will outline my setup and then detail the listening problem. The Amp is a Dennis Had Inspire 45 tube amp with brand new Emission Lab tubes, the Preamp is a Dennis Had LP3.1 with NOS tubes, Turntable is a Dual CS-5000 with an Ortofon cartridge and Sutherland KC Vibe Phono Preamp, Streaming is with a BlueSound Node, CD Transport is a Cambridge Audio CXC V2 with a Cambridge Audio DacMagic 100, Speakers are Polk Audio Monitor 10B, Interconnects are Monster Cables I’ve had for years, and the speaker cable (THIS IS WHAT I AM REALLY INTERESTED IN) is DIY from Amazon Basics 16 Gauge 1.33 mm with Amazon Basics Spade Terminals. The speaker wire is approximately a 16’ to 18’ run.

My listening problem: When I start listening to music FROM ANY SOURCE, either immediately or in roughly 10 minutes, all of a sudden the Right Channel Speaker volume drops significantly...to about half of the Left Channel Speaker volume. Then after a while, the volume equals out again. As I continue listening, sometimes the volume in both speakers will then suddenly reduce by approximately 25% to 30% -- then after a short period increase to the original louder level. Sometimes the volume stays the same level in both speakers for the rest of the listening session, other times the Right Channel Speaker will decrease again.

Before hooking the Amp and Preamp up in this current listening area, they worked perfectly and did not have this issue. When I flip the speaker cables the issue flips to the Left Channel Speaker. The tubes checked out as OK when the issue first started, and the Emission Power tubes in the Amp are brand new out of the box. I had this happen with another Tube Amp I own, and the first item in my troubleshoot checklist was the problem...a bad Power Output Tube. Bought new Power Output Tubes from Brent Jesse and problem solved. So that was the first thing I checked but the Tubes checked as OK -- plus the Amp Power Tubes were brand new out of the box $800 Emission Lab 45 tubes which are known for their quality.

Sorry for the long lead in, but I have seen from other posts over the years that Members like to have a lot of information about the gear and troubleshooting already done. So I’m finally getting to my question: CAN THIS VOLUME DROPPING AND SUDDEN INCREASES AND DECREASES BE CAUSED BY THE SPEAKER CABLE? IS AN 18’ RUN TOO LONG OF A RUN FOR 16 GAUGE CABLE? I know the first thing most of you have thought, "Dummy, why don’t you just connect another pair of speaker cables from your Amp to the speakers and see if that fixes it!" A few reasons: Currently, the speaker cable is heavily fished throughout a custom designed built-in bookcase/cabinet and is rather difficult to access. I’m partially disabled from back issues and degenerative joint disease, so I can’t easily reach the back of the Amp to run new speaker cable to test that theory. In addition, I don’t have 12 or 14 gauge cable that is the length needed, and don’t want to buy the new cable if that can’t be a possible reason for the problem. Without completely breaking the gear down, I’d need about a 10’ run of newer gauge cable just to connect to the back of the amp and stretch the speaker cable across the cabinet to reach the speakers to test the Gauge theory.

If you tell me that yes, indeed, the speaker cable could be the source of my problem, I don’t mind buying a 100’ spool of 12 or 14 Gauge cable off of Amazon and creating another long run set of Speaker Cable. I already have a box of 8 Gold Plated Banana Plug terminals, so I can build another set of Speaker Cables. If you tell me that, no, speaker cable can’t cause this issue, that leads me back to having to try and test the tubes and hope that is the issue and not that something is going on internally with either the Amp or Preamp. If I do have to reexamine the tube theory: I know on the Amp the power tubes are the new Emission Lab 45’s. I have the original power tubes that I replaced, so switching that out to test isn’t an issue. However, can you tell me which tubes on the Preamp might be the culprit? I do not know the tube arrangement on a Preamp -- such as does it have power tubes and rectifier tubes like an Amp? I’ve only owned the Dennis Had gear since last May, and as with most Dennis Had gear, there is not a lot of paperwork or manuals. He has responded to a couple of emails I’ve sent, and he has invited me to come to his garage to look at a Phono Preamp he is building (Dennis lives in Cary, NC, and I live in Winston-Salem, NC, which is only 80 minutes away). I thought I might get quicker answers on this site.  

EDIT (I FOUND THIS INFO IN A SOLD PREAMP AD):  This preamp is the best preamp I've ever had--bar none. Will ship with the original Sylvania 6sn7 tubes Dennis included which, incidentally, are the best 6sn7's I tried, beating out many higher end tubes; additionally, I will include another 6sn7 in the place normally reserved for a single 6bx7. After the 6bx7 died, Dennis recommended I replace it with the 6sn7 and I preferred it to the 6bx7. It will ship with the original EH 5u4 rectifier, which I found to be a great rectifier, plus two 6dn7 tubes, which sound fantastic in place of the 6sn7.

Thank you greatly in advance for any help. It is very frustrating to finally get this extra listening room set up in my Den with a "nice to me" Amp and Preamp, but not be able to appreciate my listening sessions!