Sound like a smart approach?
Yes, attempting to compare all three with your own ears is the smart approach. I haven't heard the Marantz, though I have heard Clearaudio models, nor do I have much time with an RP6; I've certainly never come close to the type of comparison you are hoping to arrange, so take these thoughts as just general impressions of similar 'tables.
Clearaudio products, to my ears, have excellent extension, detail retrieval, and spatial properties--music can come across as quite vivid on a Clearaudio design. However, they fail to engage me with the music like some other designs do. Another poster decried the use of terms like pace, rhythm, and timing, but I have to confess that those are the most important areas for me. I can enjoy a closed-in, airless, spatially constricted, and dark performance as long as the music has good pace, flow, and is rhythmically intelligible.
The Technics SL1200 does not have the spatial and detail recovery strengths of the Clearaudio, but I find it does have better timing and marginally better rhythmic integrity. For that reason, I would prefer the Technics to the Clearaudios I've heard.
Unlike some others in this thread, I really like even the bottom-of-the-line Regas. The Technics may be better at timing, but the Regas are among the most rhythmically adept turntables on the market and exhibit better pace also. In comparing an SL1200 to the Rega P1, the P1 was easily the more musically engaging 'table to my ears. Michael Trei of Sound and Vision compared the P1, Music Hall MMF 2.2, and the SL1200 Mk II, and found both of the entry-level belt drives to sound better to him than the Technics.
http://news.cnet.com/8301-13645_3-20005951-47.html
http://www.soundandvisionmag.com/article/face-best-turntable-budget?page=0,3
I'm hoping, of course, that the Marantz you bought proves to be everything you hoped for for this turntable purchase. If it shares the strengths and weaknesses of the Clearaudio 'tables I've heard, that's not necessarily a bad thing because many vinyl fans live for just those attributes that Clearaudio excels in. My foregoing comments were in no way meant to say I think the Marantz might be a "bad" choice; they were simply intended to point out that turntable designs differ in how they present music, and that different people can prefer one style of presentation over another. As you're hoping to do, you just need to audition the ones you're interested in to discern which has the balance of strengths you prefer. It will be very interesting to hear your comments and conclusions if you get a chance to do that.