For HT, I wouldn't concern myself nearly as much with matching the surrounds as the front 3.
Whatever it takes to match the front 3 as best you can. Call/email PSB for their advice on what matches best from their current stuff. Also, bookmark Audigon and ebay for something that might fit well.
If you can't match with the same identical speaker as main, then go within the same line. If you can't go within the same line, go with the same brand.
Its all about matching up front. I myself use PSB in the HT, and I found a great deal on a Platinum center. Those who knew better talked me out of it, and I got the Image center instead, and am pretty satisified overall, for now.