The PV118 is a 400w program cabinet. The PV1500 is a 300w @ 8 ohms per channel amplifier. As others have said, they have felt the same as you. Equipment-wise, you're underpowering your speakers - there's not enough power to drive them properly. As others have said, these are entry level cabinets - but with the right power driving them, they might sound better.
PR15's are also 400w cabinets. If you're running 2 in parallel per channel (800w @ 4 ohms), you're drawing 500w per channel @ 4 ohms, also underpowered.
The PV 1500 has 'low cut' buttons - used to cut freq's below 40 Hz and 150 Hz xover buttons. The manual has a great depiction of how to use low cut / xover features. In stock configuration, you're sending a full range signal to all speakers.