Upgradying Tube Preamp

I currently have a AES AE-3 DJH Preamplifier linking together my Metrum Hex DAC and Pass Labs x150.5 Amp. I think the pre is obviously the weakest link in my chain, but at the same time I'm reluctant to upgrade it. I guess I'm of the school that a preamp doesn't matter as much as the amp or the DAC. Could anyone share their opinions on this? In what ways (sound qualities) could I really upgrade the Pre, and if so which ones would you recomend?

Thanks