by putting pedals in front of the HD500, you lose the ability to use the VDI connection between the Variax and the HD500.
This is actually a very important point. This could be totally subjective, but I do notice a difference in the quality of signal when using the JTV>VDI cable going to the HD500, which I L6Link to my DT25, vs. using a 1/4" into the HD500.
Essentially, you skip an entire analog to digital conversion process, and the ability to achieve dead silent hum free sound is only an option when using the VDI. Previously I had literally programmed every single patch in my HD500 to have a noise gate on, first in the chain, on all the time (my 100+ year old house/apartment has never played nice with my electronic equipment..)
With my recent purchase of the JTV and DT25, all that went away, and I have basically started from scratch with patch building, not using the noise gate anymore.
This may seem like a subtle issue, but when using the VDI, even when using the mag pickups on the JTV, the Analog to Digital conversion is occuring in the JTV, instead of in the front input of the HD500.
Also, while I haven't utilized this feature as much, you may want to - when using the VDI cable, you can program your HD500 patches to change guitar models on the JTV as you switch patches on the HD500.
I am with you on the "get two DT25's", and the idea you had about setting up dual amp chains, and then putting alternate FX into the analog FX loops of each DT is a great idea! Especially since with the programming of the HD, you can literally tell one DT to become as you said, a Marshall and the other a Fender. As soon as I get done paying off the DT I just bought, I will definitely look at getting a second one..
As far as analog pedals go, I only have one word: KEELEY!!!
I used to have a lot of fun with my Keeley 4-knob comp in various parts of the HD500 signal chain, in front, in the FX loop, in the amp fx loop. Great stuff, some of the best pedals you can get. Now with my current rig, I have loaned the Keeley 4-knob comp to the other guitarist I jam with, his rig is all analog pedals, and a Fender Tweed.
You can do some amazing things with an amp like that, and Keeley type compressor in the fx loop. Turn the master volume on the Keeley all the way down, and then turn the amp drive and volume up louder than normal - a typical 40watt Fender like that is usually too loud around 2.. So turn it up to 5. Then dial up the Keeley master volume *slowly*, also dialing up some compression.
This allows you to drive the tubes hard, use the compressor to keep your volume even, and keep the speaker volume where you want it. Turn down the guitar volume knob, and instead of less volume, it just "cleans" up your tone, at around the same overall speaker volume. Brilliant stuff. I heard they are doing a Keeley bass compressor too!