If you're getting a big volume drop when active and bypass when using KillDry, you're send/return levels on your amp may be mismatched, or the instrument/line switch on the pedal may be set incorrectly. With my DualRec, I usually set the send and return levels to 50%, set both switches on the pedal to Instrument, and turn KillDry on.

Another option is to set the amp's Send to 50%, the amp's Return to 100%, both pedal switches to instrument, and KillDry off. Setting the amp's return control to 100% makes the loop perform like a series loop.

Goldgolb hit the nail on the head with the explanation why.