I'm like you - I also use vintage tube amps without effect loops (Hiwatt, Gibson, Fender) together with a couple of stompboxes and an EDP - it works great for me! I make sure that I put one (not more than two) buffered stompboxes with more or less clean buffers (like analog Boss pedals or TC stuff, not Nobels, Danelectro e.g.) before the EDP in the chain to convert my guitar output into low impedance, the other pedals are true bypass. I also had the input section of my EDP customized by a well known guitar tech here in Germany called Dirk Baldringer who makes a system called the "effects legalizer" that all the big time rock/studio guys put into their "fridges" over here. I don't know how he does it but he certainly cleaned up the EDP's signal, also setting of levels is much less critical than before.
Two other workarounds I can fully recommend for just above 100 bucks:
- the Barge Concepts pedalboard loopboxes with BLEND function (wet/dry mix for the effect you put into the loop). I use their VB-jr. for my Boss reverbs and delays that tend to add digital noise or phase alterations to the dry signal. Works great with compressors and other stuff, too! Uses high quality components and was made specifically for this purpose, very low noise floor.
- the Lehle P-Split splits your passive, low impedance guitar signal using a top quality transformer. Has a ground lift and a phase switch. That way you can send one signal to one amp and the other via the EDP to another amp (or a second channel of e.g. a Hiwatt, a Fender Vibrolux etc.etc.)
Of course a digial setup has a lot of interesting applications but it will NEVER, EVER be able to substitute for a well maintained tube guitar amp. A rented Fender Twin with chinese tubes is another story, but still better in most cases.
In fact, at the 2008 Berlin Loopfest Leander asked everybody to use the PA for my set and I took the plunge and plugged a couple of stompboxes followed by the EDP into the house via a G-Tronix Stimulator DI-box w. speaker simulation (similar to the more popular Palmer devices, but smaller). I did work fine more or less and reduced schlepping quite a bit!