I did some digging for you. At least on the original Sovereign model (Prior to the XS) the jumpers marked LK3 and Lk1 on the board are for fixed or variable tone from what I can dig up on that. I would guess you'd jumper those two together to put it in fixed tone mode, as the pic below is for an XS2A which appears to have no jumper on LK3 (if that is LK3 from what I can see) to any other (I would assume LK1 it would be jumpered to?). The pic below was talking of the silent search mode jumper on an XS2a and not fixed/variable jumper, so use my assumptions here with caution unless somebody can confirm it. I think both these sumpers (one for silent search and one for fixed or variable tone) might also exist on all Sovereign circuit boards up to and including the GT as I vaguely remember reading that somewhere, but never paid attention to that when I've had my GT open.
Also, this pic below will show you what pot adjust the pitch of the targets if you decide to go that route. Again, iIt's a shot of the XS2a board but again probably the same setup on other models. If you do tinker with that be sure to mark the POT at it's factory setting so you can return it to normal if you don't like what you've done, or also for resale reasons. Nobody is going to want to buy a Sovereign's who tones are "off" from all others from the factory. Not sure what the threshold pot marked is for. I would guess it's to adjust it's window of range so the external pot has the right window of adjustment maybe? Wouldn't think it would be to adjust the pitch of the threshold since we all know the threshold changes with last target it and holds that sound.
Back to the fixed/multi tone thing...If you look close I think there is an LK3 marked jumper pin at the right of the silent search jumper they marked on the board. Can't tell easily from the pic though. Better to wait for somebody to confirm any of the above stuff regardless of which route you go as what I could dig up was scatchy in content without much in way of detail or specifics. I've never felt the need to want fixed tone on my GT, and the GT already has a silent search switch so I didn't need to look into that either for the odd situations when I want to use it.
[attachment 251600 2aboard.jpg]
Again, I'd wait to confirm from others who have tinkered with this stuff as I haven't on my GT. Also, before going the fixed tone route I'd try a mini plug in equalizer at Radio Shack to try to adjust the tones to your hearing abilities, and if that doesn't work then if it was me I'd play with the tone pot on the board before I'd give in and go fixed tone. The multi tones of the Sovereigns are one of their biggest weapons. Most people have either low or high freq hearing loss. By adjusting the POT on the board I bet you can get stuff to sound to your liking/abilities. It sounds like you have low freq hearing loss having trouble hearing low tones. Raising the pitch of all targets via that pot might be the ticket. Just use a coin though to raise it only as far as you like it for coins, because that POT will move all targets higher or lowering in sound with each other as a group. Raising it for nickels will raise it for coins, so I'd use a coin to see how high you can tolerate coin hits and then check and see if nickels are then good for you now.
I think Ron was recently talking of liking raising the pitch for nickels and such a bit, and I know some raise the pitch on coins as they feel it allows them to hear them at depth better. Doesn't increase depth in the machine, but it in effect does that if your hearing is better at depth with a higher pitch. Before owning a Sovereign I had my ears tuned to the somewhat higher pitch of the QXT Pro on silver and almost adjusted the POT on my GT to match that. Over time I've got used to the GT's pitch now which is just a bit lower than my QXT's high tone was. However, I think I still prefer a bit higher pitch on all targets (coins, nickels, foil, etc) to my hearing's liking better, so I might raise that POT and see one day here. I'm thinking I've got low freq hearing loss because I have trouble with hearing people whisper, yet I seem to be able to hear the faintest twig snap in the woods from a distance when deer hunting.
PS Reminder- Get any info above confirmed first before tinkering. I only just now dug this up and it was sketchy at best what I could find.
Good luck and let us know how it all pans out...