I think your headphones are quite sensitive, so they are picking up the noise floor of the amps. Also, you probably have more gain than you need so you hear the Foreplay noise floor as well. The solution to both problems is the same - some attenuation at the Paramount output, and some more at the Paramount input. (See my white paper on signals and noise on the Community page for more depth, if you have the patience... :^)
The first check is to be certain which noise comes from which source. Short the Paramount inputs (make some shorting plugs if you don't have some, they are quite handy if you do DIY electronics at all) and listen to the headphones again. This will eliminate any Foreplay noise.
Assuming you still have some residual noise, you'll want an attenuator on the Paramount output. The noise floor of a Paramount is about 7dB greater than a SEX amp, according to my notes. The maximum output of the 300B Paramount is about 6dB greater than the SEX amp, so the signal to noise ratio is about the same for both, at 83dB. Therefor, in theory at least, all you need is 6-7dB attenuation of the headphone output. I would use a simple pot to make it adjustable; a linear pot would give 6dB attenuation at the midpoint which makes a fairly repeatable reference, and this way you can change the setting if you use different headphones. A pot of 250 ohms would give 125 ohms equivalent series resistance at the midpoint to match the SEX amp's IHF specification, or you could use a lower value (maybe 50 ohms?) to give a low drive impedance to work with a wider variety of headphones and still be large enough to not interfere with loudspeakers.
Now you can re-connect the Foreplay. If it again produces too much noise, an attenuator at the Paramount input will be needed. I have some 12dB inline attenuators in my Paramounts which worked fine until I swapped in an Eros; now my system gain is too high again and I need more attenuation. When I do that, I'll probably put a 100K pot next to the 12AT7 socket, upside down and attached with a dab of silicone goop, so I can adjust it again next time I make a system change.
Kind of a long-winded post; I hope it helps!