bose-x
Range like that from a VOR recever would have me checking the antenna system from the box right back to the antenna I am guessing that you are using one antenna for both boxes via a diplexer.
I would expect the GNS430 to lock on before the MX170 as the GNS 430 is a more modern bit of kit, this is what leads me to think that the aircraft instalation is not up to muster, it is only likely to be a dirty or corroded connection but your comments about performance changing with the same VOR from day to day I would be minded to check for the ingress of moisture into the conection at the antenna end or corrosion affecting the antenna grounding.
At one time I was running two King VOR,s from the same antenna and the KX165A always locked on before the KX155 due to the 165A,s more modern receiver, having replaced the 155 with another 165A the lock on range is identical.
From memory a think that I have receved a good signal fron SAM well south of Cherbourg at FL 70 but I will have to check this as the range did not seem unusualy low at the time I did not take a lot of notice.