how do i get more bass w/o adding a sub

My system consist of Art audio VPI tube pre, Primare a30 amp, ps audio dac, ps audio p5 power conddtioner, Jmlab cobalt 806 and just move to Usher x718. Cable is KImber silver streak . I just dont get much bass. IS there some thing i can do? This system is for my work desk room size 15x15
Try this
Well I'm not familiar with your speakers but have you tried putting the speakers as close to the back wall as possible? Maybe more or less toe-in.

That being said I just recently had the Nordost Norse Series2 demo case of cables. I currently have the original Frey Sc and the Frey2 and Tyr2 have a lot more weight in the low end.

I suspect that a significant contributor to the problem is the impedance match between your preamp and amp. The Primare a30 amplifiers have an input impedance of only 10K. The output impedance of the Art Audio preamp doesn't appear to be officially specified, but I found a thread in which someone indicated that it is 2K, which is too high to be ideal with a 10K load. And assuming that the design uses an output coupling capacitor, the output impedance probably rises to values that are significantly higher than 2K at deep bass frequencies, which would result in rolloff of the deep bass with a 10K load.

If your PS Audio DAC is the Perfect Wave, which has a built-in volume control function, try connecting it directly to the power amp, and see if you get better bass. If you do, and if you still want to use the preamp, you could insert an external buffer stage, having high input impedance and low output impedance, between the preamp and amp. Other A'gon members have highly recommended Tom Tutay of Transition Audio Design in Florida as a supplier of reasonably priced custom-made buffer stages.

-- Al
Apart from the issues raised by Almarg, I would think that you might improve the bass response some by finding and taking advantage of bass frequency nodes (or avoiding nulls). This can be more easily done using a SPL meter at the listening position and playing a disc with bass tones at 1/3d octave levels, measuring (and charting) the bass response as you move speakers about. Finding a position with a bass node about 80hz (60 to 100hz) can be a blessing for small speakers if the location doesn't produce a corresponding null at a higher frequency as it often can do.
Almarg is probably right. It's your pre/amp combo mismatch causing this. If you, demo a good integrated to see how much it helps. I can tell you the NAD C390 has fantastic bass and a built in DAC.
Room treatments.... Bass traps behind the speakers, and so on.
If your amp is plugged in your power conditioner, try plugging it direct into the wall socket. The silver streak may also be adding brightness. Test that out by substituting some PBJ cables or similar all copper.
Set up your equipment properly -- buy Jim Smith's Book. Your speaker set up is critical and time consuming.
Learn how to use your speakers, how to place them. A couple of bass traps on your room corner should help.