I have a pair of QLS-1 I restored so I know a few common problems and their solutions.
1. The EMITs become high impedance and therefore quieter. You must re-melt the "soldier buttons" on the diaphragms. You'll need a pair of 6/32 1 1/2" machine screws to safely disassemble the tweeters. Take out two of the pan head machine screws at diagonal corners from each other, and thread in the 1 1/2" screws in their place. Now remove the four remaining pan head screws and the hex head wood screws that mount the tweeter to the cabinet. CAREFULLY separate the two halves of the tweeter to expose the plastic diaphragm with the little metal "racetrack" on it. Find the two little "soldier buttons" on the back of the diaphragm where the race track connects to the speaker wire. Test them for resistance, they should be about 4.7 - 5.2 ohms, if they are higher they need their soldier buttons reflowed (if they are "open" the racetrack might be broken, which might be fixed with an automotive window defroster repair kit.) Using a very well tinned, low power (15-23 watts), iron with a fresh tip, Just touch the buttons until they give, then IMMEDIATELY remove the iron. Check for resistance and repeat if necessary. When an acceptable resistance is achieved, use the 1 1/2" screws to reassemble the the tweeters, be sure to get all the spacers and other parts back in the right way, and be careful of those super powerful magnets! The long screws hold the parts in place while you close the two halves and re assemble the tweeters. This really helped my speakers to image.
2. The pots in the crossover are known to get very corroded, this can make to drivers sound muffled or cut out completely. They are pretty easy to clean however. Take out the Watkins Woofers, this is a good time to test the woofers too, should be about 4-5 ohms on voice coil 1 (red wire) and 2-3 ohms on vc2 (black). Then draw or photograph the crossover for easy reassembly. Unsoldier (desoldier?) the wires from the pots and remove the control knob and mounting nut. The pot will come right out, it is held together with a bale, by flipping the bale wires to the side, you can open the pot. Clean it out with some DeOxit D5 or the like and a soft toothbrush. Don't use rough sandpaper of anything that might scratch the wires, this will just promote more corrosion, if it's really crusty, try a small brass wire brush. Reverse the procedure to reassemble.
3. Electrolytic caps have a finite life span because the electrolyte drys out. Replace any black and red caps you find in there, they are DONE, every single one in my speakers was leaking crust!. Big caps, like soda/juice can size, last a long time, the yellow oval caps are film caps which have a very long life, they are part of the "Infinity Sound"; leave them alone.
4. Check for air leaks. I had a bad one around one of the mid-bass couplers. It would make a weird noise when the speaker played a certain frequency.
I hope this is clear enough to follow, if not please reply with questions or comments. These are such nice speakers and everyone here at the Infinity forum wants to see you bring them back to their former glory!
RIP Arnie Nudell