Need a preamp for my sub

[badboy6120]

Hi
I have an amplifier with these characteristics:

POWER RATING at 1KHZ with 0.1* THD = 200W/ 8 ohm, 260W /4 ohm
IMD= *0.008*
THD at 200W into 8 ohm *1 KHZ= *0.005*
THD at 200W into 8 ohm *20 HZ....20 KHZ= *0.05*
FREQUENCY RESPONSE [at 50W/8ohm, *0...-3dB]= 1.5 HZ.....125 KHZ
INPUT SENSITIVITY= 1Vrms
ΙΝPUT IMPEDANCE= 48K
SLOW RATE with input filter= 20V/uS
=SIGNAL TO NOISE RATIO [at 1W/8ohm] *99dBA
= DAMPING FACTOR at 8ohm [15HZ...25KHZ] 160

and I use it after a low pass filter ( Linkwitz-Riley cross over calculator ) as below:

http://img855.imageshack.us/i/69322847.jpg/

R= 8.2K
C= 100nf
2C= 220nf
Cut off frequency is around 150Hz

I'm very willing with this filter and amplifier ( with my headphone output of my computer ), my only problem is that when I use this sub woofer with another sound source that has a lower output than my PC
So I need an excellent preamplifier so I can use it before my low pass filter, so I can get a great sound from my sub as I do with my PC

thanks