Head Alignment
One topic that hasn't been covered here, and that needs to be, is proper head alignment, especially on machines that have been moved around a lot.
If one channel sounds better than the other, or your high end is rolling off or non-existent, you should probably look at the alignment.
RTR's typically have three screws per head for adjustment. One on top, one near the tape path, and one to the side. All three can vibrate the head out of alignment.
Any tape recorder lays down a magnetic field like a barcode across the tape.
Visually, like this: IIIIIIIIIIIIII IIIIIII IIIII IIIIIIIIIIIIII.
Your playback head's gap must be aligned at 90 degrees to the tape to get the best sound: [] During playback, the I travels through the [] to produce a signal.
If the head is misaligned, // the head sees less of the signal. As both the head gap and the signal field are microscopic, it doesn't take much misalignment to degrade the signal.
How to adjust:
Ideally, you'll want a reference/alignment tape, but any "known-good" tape will do. Pre-recorded tapes, or those recorded on another deck, are better than old tapes from your deck. The point here is to get a nice string of IIIIIII , rather than //////.
Clean the heads before adjusting.
First, do the playback head.
Run your tape and adjust the bottom screw (closest to the tape path) until both channels produce equal volume. If the head has a severe mis-alignment, you'll hear the other side of the tape coming thru. This is usually only noticeable between songs or on very quiet passages. If you hear this, you'll need to adjust the side-side alignment. After getting equal levels, adjust the top and bottom screws equally until the other side goes away. This moves the head in and out until it's aligned with the signal path.
Next, adjust the azimuth, or side screw. Play the tape and adjust for max treble response. Azimuth can affect the side-side adjustment, so you may have to tweak a little to eliminate the back channels after you've done this.
Once you're done, a little nail polish on the screws will help to hold them in place.
Now get a blank tape and some good source material (CD or vinyl) and align the record head for best playback, same process as the playback head.
The erase head is usually fixed, so you probably won't have to worry about it. But on the topic of the erase head:
If your play and record heads are really out of whack, you can use the fixed erase head to get a reference signal. Record a tape on both sides, source doesn't matter. Something loud. Run it thru the machine again, recording with the level at 0, and listen for the back channels. If you can hear them, your play head needs to move side to side a little.
With any of these adjustments, small movements will produce big results. There's no need to bust out the power screwdriver, 1/8 of a turn will probably be more than enough.
Now that you're aligned, thread your favorite tape in there and kick back
EDIT: quick-n-dirty way to check the alignment; look at the leader tape.