I hear so many people talk about things like getting new stereos with active capabilities, built in bluetooth, more equalization features, time alignment, etc. built into them, and most brands have HD tuners or satellite tuners, CD changers, even sometimes their own equalizers or crossovers specifically for their units, but are there add-ons for stereos for any of these things that are like universal? I know that anything universal means that you wouldn't be able to run the device through the stereo itself, but that's not important to me. I know there are tons of universal equalizers on the market, and lots of crossovers (although I have trouble finding good ones, mostly just low-pass crossovers for subs, and high pass crossovers which don't separate the mids and tweets).
I'm just wondering if anyone knows of any universal accessories that will let you play HD radio or satellite radio on almost any stereo through a headphone jack or if there are good surround sound processors, sound field processors, active crossovers, etc. that are more or less universal. Or in most cases is it more practical or cheaper to just get a head unit that has things like this built in? Seems like lists of things like this would be useful, unfortunately I can never find them when I want to. This forums needs more stickies like that!