#1
I need to run my signal from the effects loop send of my tube amp into a series of effects that use balanced inputs/outputs, then pack into the effects loop return. I know I need a DI between the send and the first effect, but what should I use to bring the signal back down to line level for the return?
#2
You Shouldnt need a DI in the chain. First you need to make sure your amp have a Balenced FX loop. Most amps have Mono FX loops,

The only thing u need are balanced cables. Meaning EVERY cable in the FX chain. If every cable in your chain is balanced, that is all you need to achieve a balanced siginal.

If your amp FX loop isnt balanced you should look for another DI with line level convesion from the Last effect back to the amp.

http://www.musiciansfriend.com/product/ART-ARTcessories-XDirect-Professional-Active-Direct-Box?sku=180636

Two of these should do the trick.
Framus Cobra
Marshall JCM 900
peavey 5150

Mesa 4x12