I had a set of Hertz Audio HSK-165's in my 451 with a 200W amp under the passenger seat before i traded it in (and stupidly DIDN'T take the speakers and amp out prior to trading it in.... still kicking myself for that). Best speakers EVER! Least, they damn well better be for $400 component speakers.
Which i had to laugh, too... cause whoever bought my 451 after i traded it in... recently traded it in at a local used car dealer, so i went and took a peek. Apparently they took the amp and the woofers... but left the tweeters on the A-pillar.... and chances are they left the crossovers, too. Also took the GPS antenna out from under the plastic surround for the head unit and A/C controls and stuck it to the far front of the dashboard. No idea why, since it always worked just fine for me where it was, and was completely out of sight.
Can't imagine those speakers sounding as good as they used to without the crossover and tweets to back them up. I just remember letting the Smart brand manager at my local dealership at one point listen to the system in my car at one point. When i turned it off, he told me he'd never heard audio so crisp, clear, and punchy in any other ForTwo he'd been in. Kinda plan on putting another set of those in my 453... just A) need to save up the money first, and B) need to make sure they'd actually fit in the doors, since they were apparently a rather close fit in the 451.
Been researching and reading the forum concerning speaker replacement and decided to go for it. Purchased JBL GTO629 6 3/4 coaxial spkrs with directional tweeters. Removed door panels, old spkrs, old spkr covers by grinding off plastic rivets with a dremel. Drilled correct holes through door panel for new spkrs, made slot cuts in the plastic raised area to insert metal u-shaped nuts, installed spkrs and covers from the inside. Will never have to remove the door panel again to work on or replace the spkrs. Smart must have installed them from the backside for ease of mfg, but sure did make it a pain for replacing. The new spkrs are absolutely amazing in quality of sound vs the stock ones when listening to good quality music from my ipod. They are much better than stock for even the cheap radio, but not as good as the Ipod. After 2 1/2 years of that crappy, tinny sound i can actually enjoy the music. Highly recommend a spkr upgrade.
After doing this once and knowing what everything looks like, i can say that i would feel comfortable doing the same thing. As long as you have the right tools including a dremel, anything is possible. I did watch a you tube where this guy took sheet metal cutters and butchered everything up, as i was cringing, but it is possible to do without hurting anything important. That said, i now think i can remove the door panels quickly and do it the preferred way in the same amount of time.