i don't think the LM3 and the RH450 are in the same price category.
price aside, they are completely different animals in terms of tone and features. there's also a HUGE price difference.
the rh450 uses tube simulation to simulate a tube pre and power amp. i'd say they are almost incomparable. comparing a markbass with a genz/eden/ea would me more clear cut. i don't think there are any amps in the market that could parallel the rebelhead, which may or may not be a good thing.
where all these brands are concerned you are definitely getting your money's worth. that goes for all boutique gear in fact. it's all down to your needs, preference and budget, all of which only you will know best
all markbass amps are not voltage switchable, so you would have to get it from either a european or australian dealer, or davis. i don't think you can really get a good price from anywhere other than USA.
the rebelhead has switchable voltage so you have the option of getting it from overseas dealers.
from my observation it is very difficult to beat davis. unless you can't wait for the stock to arrive, it is usually better to get it from them.