Which is the best preamp for my amps.

Hi, I'm using Warner Imaging ER-300M SE Monoblock amps which sound great. I was using a NRG Preamp which is a great preamp. But Its hard to get replacement parts for it. Out of these 4 preamps, which would people recommend? Threshold T2, Klyne 7LX 3.5, Levinson 380 or the Jeff Rowland Synergy preamp.
2)Jeff Rowland

I would recommend you try all of them and not rely on anyone's opinion; you may regret it if you do.
I agree with Timo. But as a Rowland Synergy IIi owner, I can tell you that, if you do audition that preamp, there are two secrets about it you need to know: (1) If you do a comparison with other preamps, boost the gain on all of the Synergy's inputs and compensate by lowering the output level with the master volume control. This improves the dynamics significantly. (2) If you buy the IIi, substitute an FIM powerlink DC cable for the stock Cardas cable between the preamp and power supply. For $600-$700 this makes another dramatic improvement in dynamics and some improvement in soundstage as well. Even with the FIM cable, the Synergy IIi is still a bargain, compared to the alternatives listed.
Wait til the NRG dies and then ask the question.
My small contribution to the 300's

Best Pre's over different price classes

Bryston BP-25.....
Mark Levinson #32....the absolute best combo I have heard
CJ PFR...good combo for lesser speakers
YBA 3-2-1-Passion....very symbiotic combo

Be aware of the 1.3 volt input sensitivity and low impedance.

BTW the single ended inputs on your 300 are about 5dB quieter than the balanced,

Rowland is Ok, Klyne have not heard, 380 bites it, Threshold not a good idea either.

It would help more to know what speakers and Cd Player you own and notable listening room conditions and musical tastes.
I've always loved spectral. and a used dmc 12 or 20 can be had for not much $$. the new dmc 15 is to die for.