Since buying the Icom IC-705 a few months ago, I have not powered-up the Flex even once. The '705 has become my do-everything rig.
One of those things is QRO operation, usually on 6 meters but sometimes on other bands as well, dictated by band conditions. I can use the IC-705 barefoot (10 watts), with the Hardrock-50 (50 watts), with the KPA-500 (170 watts) or with both outboard amps for 550 watts.
It's nice to have all those options at the flip of a switch(es) but there is a better, less cluttered way. I'll keep the Hardrock-50 for portable operation but the KPA-500 and the Flex are now packing their bags.
Enter the Mercury III S - a solid state 1200-watt amplifier in kit form.
Ten watts in provides 800 watts out and the kit costs the same as an assembled KPA-500. Reviews for the amp on eHam are absolutely stellar. Imagine an easily portable, POTA-friendly, RV-friendly IC-705 QRP rig that becomes almost a kilowatt at home.
There are at least two other LDMOS amps on the market that will provide near-kilowatt levels with a 10-watt drive. One, from Germany, is problematic (and no longer being sold in the US by DX Engineering); the other is significantly more expensive, especially with shipping from the UK.
In addition to the items listed in the comparison the Mercury has a bandscope that shows an oscilloscope-view of the transmitted waveform.
There are other differences between these amps as well, most notably that the Mercury is not capable of QSK CW like the KPA-500. But I don't use my IC-705 in full QSK so it's not a priority.
IMD for each amp? I have no idea. It would be helpful if a Sherwood Labs-like comparison existed for transmitters/amplifiers. Even the ARRL's review of the KPA-500 (QST, Feb 2012) doesn't include this information or any spectral plots.
A recent QSO with Jim KD5JDS in New Mexico reminded me that I'd heard of
this amp once upon a time, meaning to re-visit the idea of looking
further into it at some point. That point has been the last three days. Jim emailed me spectrum plots and oscope images showing the functionality of various protection features built into the amp and its harmonic suppression on all bands.
An email from KM3KM informs buyers (before the purchase commit) that any kit ordered today has a "shipping lead time of Four to Five Months or sooner".
I'm hoping the or sooner part will apply. Other recent buyers report having to wait only 1/3rd the estimated time for their kit to arrive.