Theta Casa Nova vs Audio Prism Mantissa

Good question. I still have about two dozen pair of mixed 6922's and equivalents and 12AU7's and equivalents left over from the Mantissa and my advice is to use Upscale Audio, Jim McShane or Bob Hamme and simply ask them what they have at the moment that are good and reasonably priced. My experience with tube rolling in the Mantissa is that solid quiet tubes, particularly in the 6922's are far more important than any small character differences among current production and NOS. I splurged on some very expensive Amperex once and they were relatively noisy and not worth the price. The 12AU7's are cathode follower buffer tubes and have far less impact on SQ.  The Mantissa came with JAN Philips tubes-both for the 6922 and 12AU7 and IIRC they were among my favorites for overall clean, dynamic, uncolored sound. I recall having long discussions with Victor Tiscarino on the subject-he recommended I stick with the stock tubes but had no objection to tube rolling. This might be helpful to you; http://raptorelectronics.com/subpage3.html 

Thanks Fsonicsmith. I'm currently demoing both the Casa Nova and the Mantissa so opening her up and tube rolling isn't an option as I don't own the unit. Trying to settle on one of these two units? Any idea of what the fair market value on the Mantissa would be as I would need to consider this as well in making a final decision? 

Are you  in Columbus Ohio if so I'm in Columbus as well 

I am and my Mantissa which is in excellent condition is for sale. i have no idea as to current "fair marker value". My ARC dealer from whom I bought my Ref6 has it;  Brad at Sound and Vision in Powell. He was going to sell it for me. If interested, contact him. No reasonable offer will be refused. I no longer have the box or manual. I would throw in my collection of tubes if wanted.