I was wondering if I bought a seperte Pre ampwould that help me getting more bass from my turntable
Possibly, possibly not. I don't think the answer is a new stand-alone phono pre-amp/equalizer.
I do relise that vinyl doesnt have too much bass compared to digital
Whilst vinyl is deliberately pressed with a markedly tilted frequency response (far more treble than bass) to keep groove width down thus allowing reasonable recording duration on each side of the record - and to allow the cartridge to track the grooves more reliably - this deliberate tonal imbalance is electronically corrected (i.e. equalized) within the phono pre-amp/equalizer.
You have found that the turntable's in-built phono pre-amp/equalizer gives more bass than the Onkyo's phono input but that's not to say another manufacturer's phono pre-amp/EQ would give you the sound you're after.
One thing that tends to happen with our music playback setups is that we grow accustomed to the sound it produces. The danger then is that the sound of the familiar system (in your case, maybe the Sonus?) becomes our reference - no matter how accurate or inaccurate it really is. I wonder if this effect might be happening here? Maybe you should just give yourself a few weeks/months to grow accustomed to the turntable sound before deciding to part with more money? No review I've seen of the AT LP5 mentions bass lightness.
If, after a while, you decide the sound is still too bass light, then I'd look at other components in the system such as the speakers and cartridge. One last point. IME, increasing the tracking weight of a cartridge, towards the top end of its permitted range, will often increase bass weight so worth a try. Even placing the turntable on different surfaces or fitting a different turntable mat or using a record clamp can alter sound quality.