The Blackface amps have 2 more gain stages on the Vibrato channel compared to the normal channel. I suspect that there is more volume discrepancy between the actual modeled preamps than is in the POD models. I've used a few modified amps over the years... A Twin with a preamp out, a Deluxe with an effects loop... And then a few different Boogie amps with modded effects loops and they all put out a different signal level from the preamp.

I can't comment on whatever the design principles used were or were not.  The PRE models are primarily designed for use with the DT-50 and DT-25.  Users without a DT-xx can use the 'Full' models if they wish.  Whichever models they decide to use with their third party amps or even the DT-xx series, they may need to perform a balancing act.  It's part of writing a preset. However, it's always been the case pretty much with any POD that the end user may need to balance levels between their presets.

Nick

That may be the case.  Balancing levels is a necessary evil I think  

I tend to turn the cab simulation off when using PRE models with my DT50 or DT25, but that's just personal preference rather than from guidance.  I should probably not do this as when plugging in to a DT-xx, the cab sims used are 'live' models and therefore the actual speakers are taken into account.  The DT-50 212 I have is equipped with a V30 which has a strong character of its own, so turning the cab sim off is probably not quite as noticeable as it might be when using a sole speaker with a flatter response such as maybe the 112 combo variant with its G12H90.  It's all a bit subjective and the bottom line for me is getting a great sound that I can work well with.  There are times when I will use the cab sim depending on what exactly I'm after.

I'd keep your original set of tones in a separate set list and take your keyboard amp to the gig anyway and maybe even wire it in so you can flip to the original amp set of tones between songs if necessary   I hear you on redoing all your DT patches, but experience has taught me not to 'fix' something that works before a gig, so you're better prepared with a known good backup system just in case s*d's law decides to pay you a visit.

Of course some of the reasoning behind the differences in levels might be traceable to the architecture of the original amps modelled.  The POD HD500 family series hardware is equipped with six controls for Drive, EQ, Presence and Volume.  Those controls more or less match a typical Marshall or Mesa amp's real controls, however a Vox AC30 doesn't have the same set of controls wired in the same way, so for consistency in terms of control, Line 6 may have exercised a little license in adding a control where on the real amp it doesn't exist - if that makes sense.  The interaction between the Gain/Drive control and the channel Volume control is necessarily different between the different real amps.  I do know the designers' aim with all HD modelling is to try and stay as true to the way the original amps work and sound as is physically possible.  When you have a specific user interface on the hardware itself it is difficult to apply something in terms of the variety of amps modelled to a 'one size fits all' hardware package.  I agree that it might be a bit of a bind having to adjust amp model levels to match, but doing so is possible and not a particularly difficult thing to do and for me personally it's not a big deal in the scheme of things.

Nick