Interesting thread. I went down this rabbit hole a long time ago, and I think I have emerged, or at least can see light at the end. I started with a Nitty Gritty, generic alcohol water mix and fine nap brush. I noticed a positive difference, but felt there was room for improvement. I added a Spin Clean before the NG, and noticed a further improvement, but still not quite what I was looking for. Records were cleaner overall, and on many the noise floor was lower the and the noise floor of my system. I could hear fade outs or gain roll off down to nothing. But I could still hear a low level whoooshh- shhhhhh sound on many LPs as the background noise floor. I read a lot of reviews and posts for ultrasonic cleaning, and the more I found out, the more I realized that US could be a significant improvement.
I recently put together a DIY US cleaner using a Chinese 9L US tank and a Vinyl Stack to spin the LPs in the tank After much research, I settled on Rushton's diy cleaning mix using Tergitol, Hepstat 256, 91%+ isopropyl, distilled water. After experimentation with times, method etc I now use the following procedure:
Clean with Spin Clean to remove all surface dirt, and most embedded dirt
US Clean for 15 min at .3 RPM (1 rev per 3 min) in 35 degree C water, using the mix described above
Nitty Gritty Vac Dry
My results are outstanding. I can hear an audible difference from the addition of the US step. Cleaner, clearer and more distinct transients. Much better HF extension, more air and ambiance around musicians. And almost complete removal of the whooosssh- shhhhhh background noise. I know it is a cliche to say that "a veil was lifted" but the additional clarity I hear is just that.
What prompted my addition to this thread, was several comments made at the beginning of the thread, specifically regarding embedded dirt that remained impervious to several methods of deep cleaning. My theory is that dirt becomes embedded, and maybe chemically bonded to the walls at the bottom of the groove and into the very small undulations representing leading edges of transients and HF content. The use of specific cleaners and the US cycle breaks down and removes this last layer of debris.
My evidence is the surprising amount of effluent in the bottom of the US tank after cleaning a dozen LPs. Remember, all records are cleaned via Spin Clean and rinsed before immersion in the US tank. The effluent represents debris NOT removed by the Spin Clean alone.
It is not all lollipops and unicorns. Greater clarity also makes some types of groove damage more audible. And on some LPs I experience an increase of pops and tics as I get closer to the inner groove. I think the later issue can be addressed by more careful drying.
Anyway just my experience; yours may vary.