Benefit of low output moving-iron Grado ?

No professional setup needed! Just swap cartridges!

I have no experience with modern Grado cartridges. However, I do own a Grado TLZ, which was one of their early efforts at making a low output cartridge. (Incidentally, I don't think the Grados are classic moving iron types; I think they are "induced magnet" types, altho that is a distinction without much difference.)
The TLZ was a great cartridge in its day. On the other hand, I also don't think that all high output cartridges are inferior to all low output ones. You have to take this on a case by case basis. I have several MM cartridges that compete with any high dollar LOMC cartridge. Bottom line: you would have to try both versions of the Grado Master and make up your own mind. My opinion: given the rest of your system, you would probably be just as well off to go for the less expensive option, which I guess means the high output version of the Master. What Photon did, buying two new phono stages to try to make his low output cartridge sound OK, is nothing short of heroic, and expensive.

The Master is a $1000 dollar cartridge. What Grado are you talking about? I thought you said Sonata low and high output!

Swapping the Sonata for the Master should be pretty painless as far as setup goes. It could need rebalancing though, as there are fewer turns of wire in the low output Master and the older Sonata bodies were a variety of Mahogany whereas the newer model 1 Grados are denser Jarrah wood and probably weigh more.

If I were buying another Grado, I'd think very seriously about getting the VPI 1.5mv output version, seems likely to be a good compromise that splits virtues of both versions.

Thanks again, particularly Photon. The 1.5mv version isn't listed on Needledoctor's website, but I read an old forum elsewhere (from 2003) that they've "not discontinued them yet". Presumably this means I could get the retip price at N.D. AND get the 1.5mv version, which sounds like a reasonable idea.
I have found out that the settings on the Grado Ph1 are simply "low" and "high", with some confusion about which setting to use for a 1.5mv cartridge (it apparently depends on whether the 1.5mv cartridge is considered high or low output!). I wonder if a lack of gain-setting increments might cause a 1.5mv cartridge to hum?
Also, after reading my turntable's instructions, Music Hall recommends having a professional install anything other than what the factory installed. There is a screwplate through which the screws pass on the top of the headshell. Perhaps this screwplate moves around, and WOULD move around once the screws were removed? Maybe I could remove them gently without disrupting the position of this screwplate? I am worried I would mess it up... and I have no protractor or anything. I do know an audiophile who passes through my town sometimes, I could ask him to help if you folks think I should. I AM a novice.
Also, indeed my cartridge is .5 gram lighter than the newer Jarrah wood models, so the rebalancing you mentioned would be required - this I think I can handle.
Positive news: the retip price is fully $80 less than what I thought it was. Realistic news: time to start saving for this stuff. Thanks, Nicky