Re: Upgrade to a Loar 600?
I suspect either would be a good choice... I've not played an LM600 but I bought an LM 700 at the right price, and i think the lack of tonebars gives it a slightly rounder/mellower sound - perhaps more suited if you want to play a range of music including classical. - (not saying the 600 wouldn't suit).
With your budget, I would trawl ebay for one going cheap, and pay someone local to do a setup on it. I managed to get mine (albiet with minor finish flaws) delivered to Australia from the US for less than $700.
I was happy to do my own setup and happy with minor finish flaw, since it gives me something to work on and reason not to be heartbroken when I ding it.
Hereby & forthwith, any instrument with an odd number of strings shall be considered broken. With regard to mix levels, usually the best approach is treating the mandolin the same as a cowbell.