I've put just about every cartridge there is that retails under $300 on my stock SL-1200 and plenty of different styli, too. The SL-1200 loves them all. I keep putting off buying a fluid damper, though. Not sure why.
I do think my RCA's have been replaced with low impedance as mine read only about 80 pF. Could also be that my tonearm was rewired, too, by the previous owner. It was babied and showed signs of "audiophile use" like a backwards CW to reduce moment of inertia and increase speed.
I love the DL-103 on my SL-1200, but I wouldn't say it bests the AT150MLX. They're very different and which is "better" to me would simply be a matter of taste. The DL-103 has a lot of air, headroom and sparkle as does the 150MLX. However, the DL-103 can sound confused due to spherical tip groove pinch that is exacerbated by the required heavy tracking forces and arm mass--it can't get out of its own way. Spherical tips on light-tracking moving magnet or moving iron cartridges do not exhibit groove pinch so noticeably to my ears.
In comparison, the 150MLX is incisive and the boron cantilever makes it very fast, indeed, but perhaps a tad thin sounding against the round lush fullness of DL-103. The 150MLX may be too aggressive for the liking of some, in which case the laid back DL-103 could be the perfect antidote. I imagine that a DL-103 retipped with a MicroLine or Paratrace is probably a perfect marriage between the two and is probably also safer for your records in terms of groove wear than a stock DL-103--but I'm not a groove wear phobe so I don't see the DL-103 as a threat to my vinyl anyway. The popularity of the DL-103 modded with an extended line contact signals to me that this is the way to go. I'll get around to it one day.
