It's the end of summer and I got pulled away from posting on AudioKarma until now. Here goes: I was given (could not refuse) two Optimus Pro 4000 speakers which I knew nothing about other than they are grandkids to my Realistic Mach One speakers. They are nearly the same exterior dimensions to the Mach One...perhaps a couple inches more interior room since there is no dividing board as in the Mach One. Also, the exterior veneer seems thinner but nicer color than the Mach One. The crossover board is nicely marked and better organized, too. To be short, I ran out of time this summer and simply replaced one capacitor from a 33uF to a 14uF. Everything else appeared to still be in good shape. If I do go back in and replace caps then that will be major surgery as today's caps will consume a lot of space not planned on by this circuit board. Sound impressions: By themselves, the Pro 4000 sound powerful, accurate with plenty of bass and so - so imaging. (Reminder: My Mach Ones have the first Videolady upgrade and thus pretty good imaging.) Personally, they are a tad brighter than I prefer but not painfully so. This is where I wish they had adjustment knobs like the Mach One. So, if I were to make a decisión between either the Pro 4000 or the Mach One I would defer to the Mach One...but only slightly...they are cousins, after all. SURPRISE: So, I decided to do some minor construction in my basement and just stacked the speakers playing all sorts of Classical music, Rock and a few soundtracks...OMG! When I cranked up the volume I realized they compliment each other and sound very much like another single unit I hadn't heard in a year...an AR9! Yes, I definitely know what AR9 babies sound like for all the drooling I've done...plus I have colleagues nearby to verify and so, because of this, I thought I should pass this information onto you kind folks. I suppose this would be a cheaper, higher sensitivity rating version of getting to that mountaintop and full bodied AR9 sound.