The DeCapos, the way I see it are sort of accurate, but not necessarily analytical speakers. What you are simply getting is that the monitors are amplifying the qualities of the SS amp you have. Synergistic combination as everyone here mentions is for call. Get the right tubed amp and you will be rocking....or if you get the wrong tubed amp, the lows will be flabby, especially if you come from the SS camp.
I for example, have two pairs of monitors that you may classify as warm, smooth, but even with these monitors, Epos ES 11 and the Gershman x-1 satellites, you can have "too much of a good thing." Thanks to the DNA 225 I have which runs ever slightly to the cooler side of the sonic spectrum, it balances rather synergistically with the X-1s and add on top of that the Final Laboratories Music 5 battery powered preamp, the whole set up just sends up the performance up.
You will never be happy simply by piecing a system and expecting one component to play up with the other. In this case, you need to make priorities, either favor the speaker or favor the amp.
Cannot win all the time.