Sure, you can bypass the volume pot...but thats the same as "full rotation" which you said worked.  Just leave it at full and continue working.  You can also use two single pots rather than the dual ganged one that comes with the kit, then you can adjust the volume independently for each channel.  You can even use Radio Shack pots if you have to have it today,  100K-Ohm Audio-Taper Potentiometer  Model: 271-1722  Catalog #: 271-1722.

You need to check one thing that would make the amp sound ok at low volumes but distort at anything over 1/2.  Be sure that pin 6 of eack output tube (shown as a n/c on the schematic) is connected to pin 3 of the same tube.  (this is an internal connection on some output tubes but not on others...if you are using 6p15p's and don't have this connection wired on the board for each output tube (or have the CCE mod installed)  then you will get distortion at anything over about 1/2 watt of output.  I believe later Zkit boards have these connections made in foil and earlier ones don't.

Be sure to read the "hum problem" thread in this support group too.

Dan

If you remove the source, the hum should go away - even at maximum volume setting.  Now if your volume pot is damaged (open), I can see how that might cause problems by leaving the grid of the input tube open with no resistor to ground.  I have always felt that 100k is a little too high here, and have used 50k replacement pots in my Zkit1 (I'm using two so I can control each channel).

If you remove the source (because this will short the source to ground) and temporarly solder ("tack") a small wire across all 6 pins of the pot you should not have any hum.  If you still have hum, you will need to find the problem before continuing as it does not involve the volume control.  Be sure to remove that wire before plugging any source back in, and (as always) be sure to power things down before doing any soldering.

Dan