ground loop buzz/whine

[dmjlambert]

I have a 1969 CST/10 with a 350 engine and GM HEI with full 12V going to it, and with AC Delco stock spark plug wires and a CS-144 internal regulated alternator stock for a 1994 pickup.
I don't have a radio in the stock position in the truck. Instead I put a Kicker amp and JBL speakers under the seat, and a Sparkomatic 8-track player under the dash. The 8-track player does not sound good on its own because it only has 3 watts per channel. I put a Kicker speaker-level to line-level adapter on the output of the 8-track player, and from there I have shielded RCA cables under the carpet to a Fosi mini-amp with Bluetooth, and the output of the Fosi mini-amp goes speaker level using 16 gauge speaker wire directly into the Kicker amp. I have the input selector on the amp set to hi-level and I turned the gain all the way down. I also turned the gain all the way down on the speaker-level to line-level adapter, and I have the Fosi mini-amp volume turned up to the max that I would want to play for the Bluetooth tunes.

With this setup the 8-track sounds quite fantastic, and I can switch it to Bluetooth and play from my iPod or iPhone and that also sounds fantastic. When playing through the Fosi using Bluetooth it sounds good and there is no noise at all. When Bluetooth is off the Fosi amp automatically switches to the aux input which comes from the 8-track. With no 8-track in the player it is quiet, there is no noise. When I plug an 8-track into the player, the player turns on and on top of the music or between songs there is a whine that varies with engine RPM. I have heard that called alternator whine by some folks. The volume of the whine does not increase with the volume of the 8-track player, so it is only annoying when I listen to something on fairly low volume.

I have made sure my ground and power connections are very good, and I even tried running a jumper over to the amp's ground point to provide ground for the 8-track player and that did not make a difference.

I did some reading and figured this would be solved with a ground loop isolator. When shopping for one, there are many to choose from, but no name brands like Kicker, Kenwood, JBL, Alpine, etc. they are all names you've never heard of. So I got one branded Install Gear from amazon.com. I put it between the 8-track player's speaker-level to line-level adapter and the Fosi amp, and it completely eliminated the noise. But it also took out all of the bass and most mid-range, leaving just shrill highs. Do any of you have experience with ground loop isolators, and are they just like this not having full range, or did I get a bad one? If there is a brand that does not alter the sound that is what I want, because the 8-track player sounds very nice, I just have that somewhat soft whine I would like to fix.

Thanks for reading my long post.