That won't be easy to do since the 2.1 system gets its power from the subwoofer module. You'd have to either go with a passive speaker pack with a receiver or spend a premium on active monitors. I don't think you will get anything decent for $300. Cheapest would be the Energy Take 5.0 speaker package plus a refurbished receiver for maybe $350 to $400 and use the existing ProMedia subwoofer. Klipsch does have their HD Theater speaker packages starting at $400.
Denon AVR X7200WA, Emotiva XPR-5, Emotiva XPA-7, Sony XBR 75" 850C, Oppo BDP-103D, Roku 4.
DefTech BP7000SC (L/R), CLR3000 (C), SM55 (FH), UIW BPZ/A (SB), UIW 75 (TM).
JBL L7 (Sur), EoSone RSR 350 (RH).
PC 2.1 setup. Emotiva Stealth 6, SVS SB2000 sub, Emotiva DC-1 DAC.