I love the PDI-09!!!!! Its speaker sim is also pretty decent, really maybe the best I've heard, surely the best non IR based.
I have to say, an FX loop will NEVER sound as clear as a W/D setup. I never tried it before but it kills!
the wet/dry route works really well and a lot of players do use it
Whilst many know that EVH does exactly this - especially as the old Marshalls had no loop and it gave him a lot of clarity
The other player who had an obsession with it is Brian May
That is why he always uses AC30s in 3s or multiples of 3
He says it eliminates intermodulation distortion within the amp that occurs when using echos and other FX.
And certainly amps that have a parallel FX loop retain a better signal when using certain FX units that seem to rob tone and dynamics when used in series within an amp's loop. I found this out using a rack - the Alesis quadraverb was a big tone sucker when placed in series after a preamp . Using a mixer and putting it in parallel massively improved my sound .
The TC Electronics units I use now do seem somewhat better in that respect and I get away with using them in series - although I keep planning on putting them in parallel again.