Input 2 is set to Variax which I'm NOT using. In the mixer, both input faders are centered and at 0dB.
The Tube Drive before the empty amp blocks sounds crisp and defined tonally; but when I place it (without changing any of the device parameters) behind the empty amp block, the Tube Drive sounds dull and lifeless...
given that there isn't any amp (neither a deactivated one) in its empty slot, the position of that slot doesn't have any influence on the signal..
what really matters in your pictures is that in the first one your overdrive is in pre split position, and in the second picture the overdrive is instead in path A (separate path since there isn't anything shared in pre position)..
as you said, you are using 1 active input (being input 2 unusued)..
I and other people in this forum have demonstrated that when using a single input the same effect in pre-split position receives a signal lower than what it would receive in a separate path A, and the ratio is 1:2, ie, the same effect in path A receives a signal of double intensity, and consequently generates a different response..
there are 2 different theories on the POD routings which circulate in this forum: mine and that of the member "perapera" ..
the only thing in common between them is that we both agree that in a case like yours with 1 active input the response of the FX/amp changes depending on its position: in pre split or in path A (separate)..
my theory is here: http://line6.com/sup...e-of-the-hd500/
and for a bit more in depth post: http://line6.com/sup...ge-3#entry44388
if you want read also the "perapera" different theory, you should find it easily in this forum..
so very briefly I'm going to explain the signal routing behaviour of your case following my theory:
- in the first picture the overdrive is reached by your single active guitar input signal, and it sounds crisp and defined as you say..
- in the second picture the same overdrive is reached by the same signal of the previous example plus a copy of the same signal, giving a total of 2 signals together going into the same block, and you get a dull and lifeless sound as you say..
- why that POD's strange behaviour?
because there is a sort of fixed compensatory mechanism that comes into play in case you need 2 amps or more generally when you split your chain in 2 parts..
in these cases very commonly happens that at some point of a single chain you create a split point by enabling 2 full stereo paths A and B (simply by putting something into one of them) starting from there..
each effect/amp block generates 2 halves L and R of its processed signal, and the last block before the split point send its L part to path A, and its R part to path B, but being L and R two mono halves they need to be doubled to have again a full level in each path, and for this reason everything going into a full stereo path gets doubled from the point where starts the split point..
this behaviour is fixed and applies also if the split point starts at the very beginning of the chain, ie when the two paths are completely separate without sharing anything..