recommendation for a high end analog system

yes, already optimized impedance and gain. If I wanted to change the tone arm on my SOTA, how would I even do that? I think SOTA has a limited arm choices.

May also want to check, double-check, and try different Impedance and gain settings on your phono pre to make sure you are getting the most from your 103. But, you may have already done that.

My friend's system is Spectral DMC30, DMA250 with Quad 2912 speakers (all MIT cables). Actually I haven't decided on my budget, but I want to get an idea of what is available.

Well, you have already started to figure out where to start, and as a SOTA owner myself, I would stick with that. As you say, your friend has the same, and his rig sounds better. So….

I believe your arm is basically a Jelco, perhaps a 750, rebranded by Sumiko. That isn’t a bad arm, at least I doubt it is the weakest point. Which points to your cartridge, and/or phono preamp, as the two pieces I would start to look at upgrading first. Or, the cart and arm, if you like your phono pre.

No doubt his Triplaner is a better arm, but so is his cartridge, and that is personally where I would start. The 103 seems to have a great following, but it isn’t a Lyra. That was a $5,000 cart (still sell in 1.5-3k range used today). Carts do make a significant difference, it’s up to you to figure out how much you want to spend to get the same/similar sound. I would start with the cart first, then, a toss-up between the arm and preamp, but I think a preamp could make a more immediate impact.

Welcome to the audio hobby!  
What is ‘breaking the bank’ and can you share the amp and speaker details you are comparing your system sound too?