Before I attempt an answer, it would be helpful to know your room dimensions, relative placement, and what speakers you've used before Zu.
I own a couple of pairs of Omen Defs and found they definitely take some time and effort in setup to get them sounding their best. The bass is greatly affected by the amount of gap between the bottom of the speaker and the floor. Even small differences (1/8") can greatly affect the amount and tone of the bass (smaller gap will result in more bass output). The other thing I have generally noted with Zu speakers is they seem to like a greater spread than other speakers I have had in my room.
Thanks for the input!! I still own dc10audio instrument 8 floor stander which are now in my living room my wife like them there and she is the boss. I have a listening room (man room) that is 24 x 32 deep with the 9 feet ceiling PLEASE NOTE The Rotel gear is the top of the line Michi These are older Zu speakers like new not the newer ones I have played the dc10audio Berlin there which where natural dynamic and produced big solid bass with only a small woofer I thought the zu would be even bigger with two large driver! My room needs help?
Are you Zu's bottom ported? are you on wood floors or carpet?
when i moved my Superflys to a carpeted room i noticed the bass totally went away because the bottom of the speaker needs to be an inch or so off the ground. (the zu site has the recommended distance) i ordered some Herbie speakerfeet to raise my speakers off the ground and BOOM bass came back. had nothing to do with my amps that i kept switching. hope that helps.
Zu Omen Defs should yield ample bass in your room, to their lower limit but keep in mind that the 10" drivers are full-range very low excursion units. They will deliver fast, tuneful bass but not with the artificial bloat and distortion of a longer-excursion woofer. The Omen Def cabinet uses Zu's proprietary Griewe acoustic impedance scheme and as others have mentioned, the height of the cabinet-to-floor gap is highly influential to the bass profile you will get. Very small differences in gap height yield large differences in bass character. Generally as you move closer to the floor, bass definition will improve but some lower limit is given up, while increasing the gap mximizes lower limit while gaining some bloom at the expense of definition. You can dial in the gap height to preference and somewhere in there is the right balance. Zu has clear directions on this.
I too do not think the Rotel Michi is an ideal sonic match for Zu, though of all the Zu speakers, the Omen series should be better for it than the rest. Top of the line for Rotel or not, the clean and capable Rotel is not strong in full-body tonal representation, which the Zu driver lays bare. You have the option of updating the older Zu FRD with the current nanotech drivers, but this may make the problem with midrange worse, as the new driver's transparency reveals more of the Rotel's essential character. You'd have to be more precise in describing what you mean by "quacky" for me to say more.
A tube amp with some discipline in the bottom end would go a long way toward optimizing Omen Def, but you may be able to find a better solid state match if that's important to you.