Its better to use any clean boost/ volume pedal in front of the amp

 So the amps OD isn't enough for you?  Curious what style amp do you have? e.i. Non-Hrm, HRM etc. ?

 Or do you just want to adjust gain levels on the fly?

 If you use a Vol. pedal in the loop w/ out a D-lator/Klein and you have it on the floor you will most likely get signal loss w/ long cable lenths. I can't say 1st hand about using a clean boost in the loop. If you utilize the PAB the right way it's sole intention was you shouldn't have any problems. It boosts gain levels and lets you cut through the mix (Clean or OD).

Check out pickmasters boost mod.

yes. It won't work without a Kleinulator or similar.

Here's the mod I mentioned. It applies to the clean channel but there's no reason you couldn't mod it to work on the ratio pot.
http://ceriatoneforum.com/index.php?topic=740.msg5657#msg5657

I've had the same thoughts about trying this - in effect what you want to achieve is the equivalent of tunring up the amps master but through the use of either a footswitch (activate clean boost) or volume pedal.
The FX return on the OTS is post Master Volume and feeds straight into the phase inverter so it's the right place to make this change. However, you cannot successfully place FX units into this loop without using a Klein or C-Lator. The basic problem is impedance mismatch where the Volume Pedal / Clean Boost unit 'load down' the output of V2B (2nd clean triode) and the input to V3 (phase inverter). Doesn't matter how good your cables are, you will have a noticable impact on the tone (reduced highs, unwanted distortion).that's my take on the scenario.

What I don't understand is: Why can't I modify this passive fx loop to be similar to my early (series I ptp) budda SD30 passive fx loop? Everything worked fine in that amp...

I'm thinking a footswitchable second master volume may be the best solution...similar to the early JCM 900 in functionality. A footswitch uses a relay to switch between two MV pots.

I constantly leave the eq gainstack bypassed ("boost") on.

Any thoughts?

I use my Rocktron Xpression to get a (clean) boost. I have 2 sets of patches. The second set is the same but the level is set higher. So with my midi controler I can switch to the louder patch. works well.
The Xpression has an internal mixer where the dry signal stays analog. Only the wet signal goes through an A/D converter.