I'm sorry, but I really can't understand djairouks half the time when he's posting. I still don't really understand him when he says "American" sound...if the amp has too much treble or whatever...maybe you should try eq'ing it differently? Maybe thats a stupid question...hopefully you've already done that...At the studio I get bands to layer my Dual Recto 3ch with my 5150 all the time, these two amps were MADE for each other. The 5150 gives the mids and smooth top end for all the definition in the sound, the Recto gives it the balls and that little raw sound that gives the tracks attitude. I love the blend. So owning both, I understand kind of what you mean. However, you said in one post "too middy" or something to that effect...the 5150 is KNOWN for it's midrange. Even if you scoop it on the amp, it's still really middy. That combined with it's smooth ass top end, thats the signature 5150 tone. Also, adding some mids to a guitar amp makes it sound more full, which is a characteristic you described as being the "European" sound that you are trying to achieve.
Maybe you should try turning the treble and/or presence down on your Single Recto and turn up some mids? Change the power amp tubes to EL34's, that will give you a more "European" sound for sure. If I'm understanding you correctly anyway. Thats my suggestion. I own an Engl PowerBall as well, and it's got a very scooped tone no matter what you do to the amp, which is cool for certain types of music, but usually realllllly heavy stuff. Like Meshuggah style stuff, where you have 7 and 8 string guitars doing simple riffs. I've made it work for a couple of punk bands that came in before, but it's difficult to use on everything. Which is completely opposite of the 5150 and the Recto. You can use them on literally anything.
I think if you're going to try for another good metal amp, go for the 5150, you can get a used 5150 signature head for like $400 on eBay all the time. You can sell your Recto for the same amount that you paid for it, well a little less but not by much. Take that money and get a 5150 signature for +/- $400. And then you can always sell that back on eBay too if you don't like it. Good luck.
~006