Which sounds best....ARC Ref 300 or Ref 600 Mk II

I have been looking for an Audio Research Ref 600 Mk II in Audiogon's classifieds. I'm now wondering if I should also be considering a Ref 300 (the original Mk I version, not the Mk II). Along with the extra 3 dB of power provided by the Ref 600 Mk II comes twice the output tubes to replace, more heat, weight, bulk, not to mention the added initial purchase cost. However, sound is more important to me and if the 600 Mk II is significantly better than the 300, then I would rather have the 600 Mk II. My problem is there is no way I will ever be able to effectively compare these two amplifiers for myself in any meaningful way before I buy one. I am hoping some of you out there have heard them both under controled conditions and will not only have an opinion as to which is best, but can also describe any significant differences, recognizing they are both fine amplifiers.
What are your speakers? I think the Ref 300 MK I's are better than the Ref 600 MK I's, but not than the Ref 600 MK II's.
Thanks for the reply, JTinn. My current speakers are the SoundLab U-1's, latest version. I love them. Which amp do you think would be the best match? What about the Ref 300 Mk II or the Ref 600 Mk III (aside from the additional cost, having no good way to hear them first, I worry about the FET's in their inputs....shades of SP-10/SP-11)? I listen to all types of music, from rock to classical. I normally chose to listen at levels that duplicate what one would hear at the same live event from a seat near the front. My room is relatively large with moderate sound absorbtion.
My goodness, without a doubt go with the Ref 600's. The U-1's will love the power. I think you should save the money and pick up a pair of the 600 MK II's. They are very obtainable on the used market at a substantial savings over new.
Thanks again for your advice, Jtinn. You don't per chance know of a good mint used pair with silver face plates do you (the pair currently listed on audiogon are partially updated Mk I's.....partially ??!!.....I've decided to pass on these and find a pair of original Mk II's)?

By the way, what do you know about the Svetlanna 6N1P tubes ARC is now using. I understand they are a high quality direct plug in substitute for a 6922 and are about half the price. I've also heard of a 6H23 also used as a direct plug in replacement for the 6922's. Do you have any experience with either of these?
Jtinn, me again. Besides Audiogon, Audioweb, and e-Bay, are there any other web sites I should be checking to find the Ref 600 Mk II's?
600 will have a faster and more accurate, more focusing sound, more a hi-fi characteristic than tuby. Really depends...