I prefer the SE to the prevous MKII, it is better to my ears, but I could easily live with the MKII (non-SE). I'm not sure how much better the 300 might be, but I can tell you this, if you are not happy with the 150SE, you will at some point not be happy with the 300. The 150SE is aboout as good a linestage as anyone would need, once they've decided that it really about the music and the holy grail equipment syndrome really does get silly at some point, and I think I passed that point quite a while ago. For the cost of the 150MKII>>SE I think it is well worth doing, but do live with it for a long while and stop thinking of the next piece of equipment, you will already own one of the finest preamplifiers available. My 2cents and MHO.