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That distorted sound when you start turning up the volume is the factory HiFi amp struggling to keep up. I hear this as well past about a third to half way up the sound quality changes and hurts my ears even before the volume gets loud. This level is still much before where the HiFi speakers would be reaching their limit.
I have read that many HiFi users have just replaced the factory amp with the JL XD600 and have been happy with that level of improvement from their stock speakers. I'm not sure on how much changing to the EPS speakers at that time is really attributing to an improvement from just the EPS speakers themselves as the amp change is now in the equation. I guess what I'm trying to say is if you're on a serious budget then I can see your point of just an amp and EPS speakers and being done. If it were me on the same tight budget, while I'm pulling apart my door panels to install EPS speakers, putting in a aftermarket front component set rather than the EPS drivers would be the same amount of work at that point. Just save up a little longer perhaps? Maybe another hundred or two?
About crossovers, if your running that JL amp you'll need to at least have a little knowledge on this as you don't want to be sending a full range signal to your front components and under seat subs. 4" mids would blow sending them a full range signal at any decent volume and subs, while they won't blow up reproducing high frequencies, they make horrible sounding tweeters. Remember that by bypassing and running a aftermarket amp like the JL, you're removing any safeguards via crossover points that were put in place by the factory. The actual x-over points will vary depending on the drivers you get. You could start at a high pass for the 4" mids at 150 to 200Hz then low pass the subs at 125hz to see how they sound. Adjust from there. Try not to high pass those EPS 4" mids any lower to make up for the inability for the subs to play at that higher end frequency or you might blow the 4" mids up.
And by the way, if I read your earlier post you say your E90 HiFi system has no rear speakers? (my 2010 E92 HiFi has rear speakers) If that's the case then you won't need the 6 channel JL XD600 amp. You can get by with a 4 channel amp. Front two channels to run the front component set, and rear two channels for the under seat subs. The benefit is not only additional cost savings as 4 channel amps are less expensive than 6 channel amps, but unlike that JL 6 channel amp, the JL 4 channel amps have a RCA pass through so if you want to run additional channels through to another amp later to run speakers or a sub you can do this without having to use a RCA Y adapter to split the RCA's from the Technic harness. I have confirmed with JL audio tech support that the RCA pass-through on their amps can then be treated like normal, un-balanced outputs so you could run ANY amp from that feed and not worry about the balanced differential signal being an issue.
Or you can stick with the 6 channel amp and bridge 4 of the 6 channels to feed the two sub channels with more juice if your upgrading those. Make sure if you do this you note the impedance of the woofers, some multi channel amps won't let you run 2 ohm bridged, only 4 ohm bridged.
Any time when your doing things on your own to save money, in turn it's more to learn. I've installed audio systems in my previous cars and now time learn again myself since our M3 HiFi system presents unique challenges. It's no wonder most BMW owners just buy the preconfigured Bavsound and other BMW specific audio kits and just plug them in and go since not only do they sound better, but they've also taken away all of the technical stuff you need to know when you start doing things on your own. There's a reason why places like Musicar Northwest are in business.
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