what impacts the sound quality more amp or pre-amp

To answer your first question:
Whichever is the weakest link.

To answer your second question:
Everything counts in the system chain, from the source, to amplification, (pre and power), to the speakers, (and even the cabling can have a big impact). System synergy is everything. So a balanced approach is best, IMHO.

It's about interface with the surrounding components. The pre needs to have enough of the right inputs and outputs, perhaps be able to handle your given cartridge(s), have an appropriate output/volume control and output impedance for the amp. The amp need to have the right input sensitivity and input impedance for the pre, and the right power output and impedance handling for the speakers/room, and perhaps the appropriate output taps and number of binding posts, and oh yeah, they have to sound good together! With all that said, and being in complete agreement with Kurt, and though you'll probably have more hands on with the pre, IMHO the amp is usually more important.

as typical we i am always looking to improve the sound of my system with a budget in mind. That being said, I was told I would get more bang for the buck in investing on the pre-amp side of things and keeoing my existing amplifier. I have a SS system using B&K components and desire a more natural or as they say tubey sound with a larger soundstage. My budget is 500 - 800. My impression is I would do better by buying a Prima Luna 3 pre-amp versus purchasing a hybrid amp such as a MOSOCODE 300???

Dvdgreco,
As others have stated above, all components within a system matter, the weak link will drag the overall performance down every time. But to answer your specific question, in my experience I`ve had more profound change in the sound/character of my systems with power amplifiers more so than preamps without exception. The transition from solid state to push pull tube and then SET-DHT amps were major steps.

I think a high quality sounding preamp can carry a mediocre amp and visa versa. But without question you will want to eventually upgrade any other inferior component(s) to get the desired sound you are ultimately seeking. I would tackle the preamp first since its job is the toughest and it is one of the hardest components for a manufacturer to get right. To find one that sounds decent within a certain budget can be difficult but not impossible.

A few years ago I owned some B&K components & liked them but found out that some of their models stand out more so than others in terms of sound quality. That would include some of their older gear.