Loss of bass, quacky sounding when Tone Knob is down...

I am in love with my partscaster '62 style P bass. It started its life as a Squire Affinity, my first bass, and several years ago I hot-rodded it in several ways inside and out. As far as the guts of the bass, I have a Fender '62 RI pickup inside, and maybe the stock Squire volume/tone pots inside. I took it to a local shop to get the electronics done and I was only 18, so I didn't take many notes on what exactly they did other than dropping in the pickup I gave them and making everything worked.

Here's my problem, that I've always had but am recently being really bothered by: When I role the tone all the way up, it sounds like a normal '62 era P bass, and really good at that, but when the tone knob is all the way down with almost no treble, the bass frequencies suddenly disappear and I get this very middy, almost Stratocaster quack sound out of it. It hasn't bothered me much over the years as I usually had the tone all the way up. But lately I've been wanting to roll it back but have been running into this issue.

Any theories on what might be wrong? Does it have something to do with the phase of the P pickup? Maybe its just a crappy tone pot? I've been baffled by this problem for quite long enough, and am just ready to have a tone knob that gives me clarity when turned up and gives me nice round bass when rolled down.

 

Any ideas what may be wrong?

 

Good suggestions. Thanks! I'll reply with what I find out.