At first, try a better amp than the D50...for example a valve amp...or T1. Than you could hear the difference better...I'm testing the L700MKII with D50 and the L300 with 252S side by side.
The L700 combo definitely has way more clarity, detail, and separation. The L700 also has way more bass texture and detail - but less bass overall, especially mid-bass impact and punch that the L300 offers. The L700 is more smooth, being less harsh in the treble, and definitely the superior headphone to the L300. The difference is quite noticeable after finally listening to them both side by side. It is especially noticeable in busy tracks where there is a lot going on, the L300 can't keep up with the L700MKII's speed. I also am really enjoying a texture to bass notes with the L700 that I've never experienced before, even with an LCD4. I wish the L700 definitely had more bass impact like the L300's punch and more extension, but they are overall seriously impressive.
Here's the kicker. I keep going back and forth between the two and can't get over how intimate the sound of the L300 is and how much I enjoy it. The L300 is noticeably lacking compared to the L700s in terms of sheer audio quality, but I feel that I am drawn into the music more and enjoy it more than when I listen to the L700s. I keep putting the L300 back on my head. I want the L700 because it SOUNDS better overall and impresses me more with each new track I throw at it, but the L300 is more musical. The L700 just sounds further away, and although it is clear it has more detail when going back and forth during sections of songs, the L300 APPEARS to have more detail because the detail that is there is perceived closer to my ear.
I can only keep one and I'm not sure which to do right now. Very tough call.
Normaly the situation should be clear. The L700 is the better of them. But if you thinking the L300 could be a only small difference to the L700...than take the L300.... it is a great pricely difference. If the price for you is no matter, take the L700.