looking for quality passive sub with 8" drivers

By the time you buy/build your custom built amp, why not save the hassle and get used Vandersteen 2W's? They have tripple 8" drivers and 300w amp built in, and are eminently musical. You are correct that the pairing of smaller drivers yields a "quicker" sub. They take the signal from the amp (speaker level input) and blend seamlessly with smaller speakers. You can find them used for as little as $600-700. They come with a crossover which you can tailor the sub to your particluar amp sending the signal. Check out their website for more info: vandersteen.com You won't do better than that.

See if you can find any Cabasse Stromoblis'
8 inch drivers, passive and very quick

If the HSU cylinder subs interested you check out the SVS site. They have active and passive cylinder subs in 3 sizes I believe. I have no experience with them but it's the first thing I thought of when you mentioned hsu.