I agree with you, it's just that I like to have the freedom to tweak my system the way I want.
Part of the problem is that we have no idea what your HTPC setup is capable of and what streams it can grab and send out to a preamp. We have no idea. But you say it can send an optical or RCA digital stream. That means decoding is going to have to take place in the preamp.
Next, using Pure Direct does not result in better sound. That is a myth. I have looked at that myself and that is nonsense.
If you want to do stand alone HPF, which I don't advise, you will have to go custom.
First of all you will need a preamp or a receiver with preouts.
Then you have two options.
You can look for vintage gear and get a good analog crossover like the Crown VFX 2, or buy a new crossover from Marchand electronics. Then you will need two, two channel power amps if you use just a preamp. One two channel if you use a receiver with preouts. So you would use two power amps from the receiver and one external two channel power amp from the crossover.
Your other option is to make your own crossovers. Parts express have a good collection of digital crossover boards for DIY active speakers. So you could select one of those and a power supply and build, or buy, a case to install it, and instal appropriate terminations. To do this properly will require a fairly well equipped workshop and significant constructional skills. However it is not that difficult. I actually designed and built an active LP crossover for a set of speakers I built that had an active isobarik bass section. So I know it is possible. What I don't know is your level of skill.
However, I have to say that you are making a mountain out of the proverbial mole hill. My experience tells me you are likely to have massively lower SQ than you would if you conformed to current protocols. My hunch is that you don't have the skill sets yet to pull this off. If you had them, you would not be here asking all these questions.
When I design a system I don't go looking on forums to ask: - How to I do X. I jolly well make sure I research it, know what I am doing, draw up a practical design, and make an assessment that I have the skills to pull it off. That is the way you go about one offs. Having been down that road a time or two, and know whereof I speak.