Hey there naruto
Blackstar HT 40 is a fantastic amp - has a serial fx loop so is great for hooking up using 4CM and also sounds good when plugging the HD directly into the fx return ( 1CM).....hell it even makes the amp modelling on my Boss GT10 sound good (lol).
I generally don't use any amp modelling when using my amp ( I love the drive tones of the Blackstar Amp itself and use OD/Dist stomps for addtional drive tones - using my GT 10 or my POD HD 400 just for fx) but I have used both my POD HDs (400 and Bean) and my GT 10 through the Blackstar using the amp modelling (both 4CM and 1CM) and was very pleased with the tones.
It has 2 channels- Clean & Drive - with two modes on each Channel ( so essentially you have 2 clean tones and two Drives tones at your disposal ).
In terms of the Clean tones (to answer your enquiry) one of the Clean modes will break up at about 12 0'clock giving a nice "vintage clean/light crunch" at higher gain/volume settings; whilst the other has more of a "Fender" clean vibe with much more clean headroom and will stay much cleaner at higher volumes.
It's a great amp to use with the HD series and is probably in your price range ( it costs around £500 - £550 here in the UK so would be about 650/700 $US or thereabouts) so i would wholly recommend it.
I am not sure what the street price is in the UK, but in terms of exploiting the HD500, having direct PA and an amp that stands well on its own I think the DT25 might be something to consider...But likely the DT50 is more in the range of the power clarity you are looking for...Either way, I think both might be just a bit over the budget you have set...about $900 US....but nothing else integrates with the HD PODs like the DT amplifiers do...and the amplifiers are great on thier own also.
All the Blackstars I have tried have been fantastic sounding amps...I don't see anything wrong with choosing a Blackstar...
I think the DT25 is retailing for about £650 in the UK (not sure of the DT 50 price) as non of the local guitar shops seem to stock them ( i think the online price is about £1100 ).........was seriously considering trying to get hold of one because, as you say, the POD HD/DT units have obviously been designed to integrate perfectly, but I had already pulled the trigger on my Blackstar before the DTs were launched and didn't feel the desparate need to get rid of the Blackstar (and my lovely Laney VC30) to buy the DT......plus I like to try before I buy and non of the local shops ever seem to have any in stock (hell, my local store has only just got an HD500 in stock!!!)
I have the same amp, and after reading this post at the blackstar forums, I was able to hook my Pod HD500 up perfectly using the 4CM with zero change in volume or tone. I strictly use mine as a pedalboard now and use the channels on the amp for drive, since it's so versatile. Check this out for the best explanation of how to set up your Club 40 with 4CM if you haven't already. http://blackstaramps.forumotion.com/t487p15-blackstar-venue-in-4cm-with-new-pod-hds
Near the bottom there's a post by a guy named johnsontylerj with a picture that shows exactly how to set it up perfectly. Only thing I changed was placement of the fx block for pre/post effect location. The Club 40 is a perfect amp for the Pod. Just make sure if you want clean headroom you have the voicing button pushed in on the clean channel so it doesn't do the boutique overdrive tone as you increase volume.
I am using a Blackstar HT Stage 60 and couldn't be happier. There is plenty of headroom and the power amp really warms up the tone when it's cranked. I use both full models and pre models as well as the 4CM and it shines using all set ups.I was using two 4X10 Power engines and while it was plenty loud I wanted the tube warmth so I tried the DT25 and couldn't get a decent tone from it after a month of tweaking. I hear a lot of people singing praises about the DT25's but to my ears it sounded flat and thin. After trying out the Blackstar everything just fell in place almost immediately.