Since the Rel is self powered it won't draw much current at all because it is really only looking at the voltage that your amp is putting out. Your amp will basically see the 8 ohm load. You should not have a problem
I believe that all of the REL subs are powered subs, so what your amplifier will be driving is not the 4 ohm impedance of the sub's driver, but rather the input to the sub's amplifier. That will be a very high impedance, perhaps as much as 100K for the speaker-level inputs in some cases, IIRC, and so your amplifier essentially won't know that it's there.
I'd just connect the sub to whichever tap on the amplifier you determine your Meadowlark's sound best with, which probably (but not necessarily) will be the 8 ohm tap.
Also, keep in mind that the high input impedance of the sub, and the correspondingly small amount of current that it will draw, mean that you don't need heavy gauge speaker wire for the connections between amplifier and sub.
You will need a custom cable with a Speakon connector; as Al says the wire can be of relativly small gauge. These will come with the sub, can be made by you or purchased on here made by a member with great feedback. The RELs should work fine with tubes. I recommend the S series subs; I have 3 myself. They differ from many subs in that they are intended to augment the very low bass below the range of the main speaker; they do not filter out the bass signals to your main speakers. I have found them easier to set up than most subs with an extended bass response.
I am speaking from complete ignorance on the subject, but I am surprised that if the sub was not powered. Let me repeat that the sub was not powered that a 7591 amp could power the sub at all. Is it possible for such amps and I do own several "golden age EL-84, 7591,7868 based amps" to do it. Thry work their little tubes off just making the mostly mid and treble range music, yes there is bass but not sub woofer bass.
You guys are great. Thanks for the responses. It might have been easier to assess had the REL specs listed the input impedance of the High Level input which Almarg rightly suggests would be around 100k ohms. Just didn't think of that. And Arh I hadn't thought about the fact that the REL design approach is only looking at voltage. And Stanwal, I have noticed that there seems to be a feeding frenzy for the older S-series subs from REL, whereas the front-firing later versions seem to loose their resale value more rapidly, particularly the T-series.