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Everything you need to know about how to calculate the optimum amount of attenuation is right here in
this thread.
The CleanRF sampler is not up to the task of providing good feedback for PureSignal. I bought one way back and I'm sorry I did, and I can't in good conscience bring myself to sell it to anyone else. It's plenty fine for feeding a scope, but for any serious purpose, like running PureSignal, it is completely inadequate. Its gain flatness over frequency on the coupled port is terrible, it's adjustment not repeatable, nor marked, and its large adjustment range leaves you quite vulnerable for damage to your radio.
The claimed adjustment range on the CleanRF 5K "sampler" is from 26 to 50dB of attenuation. 62dBm (1500W) - 26dB = 36dBm, or 4W! If you accidentally did that, it's quite possible you hurt something. You would know because your radio would be deaf on RX1. The first device in the chain that is very sensitive to high RF power level is the 31dB step attenuator and it's maximum RF input rating is 30dBm or 1W. Rob's suggestion of checking performance against RX2 is a very good one!
At 1500W and 26dB coupling factor you need at least another 30dB of attenuation. At 50dB coupling factor you need at least another 6dB. That's a huge range, and all it takes is one little bump of the adjustment on that CleanRF unit and you could cook something. You could go with the 30dB of extra attenuation and turn the coupling factor to maximum (26dB), that way if you accidentally bumped it power levels would only go down, not up, but you'll find that you still have problems because the CleanRF design does not provide exactly 26dB on every band.
I'd strongly recommend you either buy an Xtronic brand coupler, as Rob already suggested, or build a proven design from scratch.
73,
Scott