I didn't suggest that KILLDRY be used in a series loop. You're right, it's for parallel loops. I suggested that the loop be set to series and KILLDRY be set to OFF.
I disagree with your assessment of levels, though. +4dB is a higher-level signal used in the US for professional audio. It's equivalent to 1.737 volts peak. -10dB is used for consumer audio, equivalent to .447 volts peak.
From the Marshall JVM410 series manual, "The +4dBu/-10dBV switch allows you to configure the loop for its use with either professional equipment (+4dBu setting) or with guitar level effects like effects pedals (-10dBV setting)."
From the TF Manual, "Most guitar amp?s Effects Loop run at Line Level so set TimeFactor?s Input and Output Level Switches to ?Line.?"
The switch on the amp allows you to select whether the loop runs at line or instrument levels and the TF should be configured accordingly.
I suggested that the OP try running the loop at instrument level, as in my experience, it seems be to quieter this way.