Bought myself a Christmas present this year. I grew up in the early 1970s and Machine Head was the default 8-track played on the way to school every day. I started playing guitar in the mid 70s and had a few bands in the early 80s. I just play for the enjoyment now.
I have seen Ritchie with both Rainbow and Deep Purple and other than Jimmy Page are my two biggest influences. I own a 1978 Marshall JMP 50 amplifier that is all original including the El34 power tubes which the amp was wired for. Ritchie's setup as you know was Marshall Majors and not many have the ability to own one of these.
I had been looking at the effect pedals from German company BSM for almost a year. They have several pedals supporting different era Blackmore sounds. I chose the RPA as it supported two sounds, the Burn album tour i.e. Cal Jam and the Machine Head sound. I have tested the pedal and we are close. I am in the process of upgrading pups to Lace Sensor Purples from StuMac Golden Vintage. These pups have to much tin when turned down. I have modified my strat to 2 pickup as Ritchie did, not mainly because of him, but didn't care for the sound of it.
There are numerous posting out there that talks about Ritchie's sound. Including Dawk Stillwell's site who was Ritchie's guitar tech during Purple era. He created the MTC component which was used in some of his guitars in the 70s. The RPA has include the mods of his Marshall Majors as well as the AIWA 1100 tape deck he used for pre-amp and echo.
Overall I am pleased with the sound and once the pickups have been changed I expect better low volume clarity. The key to this pedal IMO is running the amp about half volume. Ritchie's sound was not on 11 as he put it the Majors would just catch fire. The overdrive was in the pre-amp and not the power amp.
The BSM RPA is $360 but you will be pleased with spending that amount. You can find it in the states at Steve's Music Center. I have not tried running it on 18v yet which it is capable of and will give you even more headroom.
Updates - Purple Sensor PUPs have good warm sound. Christmas 2014 Santa gift is a Weber amp attenuator. This should allow me to run the Marshall at optimal power without melting the cats or my neighbors.