I am enjoying my analog system, but what can I do to improve?

@chungjh , You should consider this an evolutionary process. Your speakers can make use of the highest quality (not price) equipment and you have many weak links or links that could be stronger. I think first is your amps. If you like Pass you need to go to the XA200.8 but it is well known in the Sound Lab community that the best amps for driving them are the Parasound JC1+ and the Atma-Sphere MA2. Either amp will wake up the Sound Labs to an amazing degree like a breath of fresh air and they will be "final" amps. Then you can work on your front end. I would start by replacing the Technics. It's tonearm is not worthy of better cartridges and for the Sound Labs you are looking at at least $5000 for a moving coil cartridge and there are many that will sound fine. Finally the preamp and I would get one with bass management capabilities so you can add subwoofers down the line.

@rauliruegas , the key with the Technics is "for the price." For the price it is a great turntable. My problem with the arm is that it is heavier and has more inertia than it has to have. This is why most if not all of the finest arms today are straight and not "S" shaped. It is not a neutral balance arm and it's vertical bearing is high above the record surface increasing warp wow. It has unnecessary contacts and removable head shells always add mass. For the money it is fine but chungjh's speakers are not "for the money" loudspeakers.

You are absolutely right that room treatment is mandatory for Sound Labs speaker but in a much different way than point source dynamic speakers. The wall directly behind these speakers needs to be deadened to prevent the back wave from overwhelming the front wave. Otherwise, because these speakers are dipole line arrays they are extremely directional and in most cases no other room treatment is required. 

Sound Labs and other full range ESLs are very amp sensitive or rather amps are very Sound Labs sensitive. Because of the ESL's wild impedance curve and reactive character many amps just do not do well on these speakers and many will actually burn out trying to drive them. They either require an amp that does not care about speaker impedance like the Atma-Sphere MA2 or an amp that can drive a 0.5 ohm reactive load at high levels without burning out like the JC 1+. High powered SS class A amps will usually do OK. Other tube amps need not apply at all. Amplifiers really make a huge difference with these speakers. I would consider in this case the speaker, amps and room be looked at as one unit, the most important one in any system but in this case they are inseparable. 

All the other issues you talk about are important and should be addressed but IMHO are secondary. I would also suggest to chungjh to have a good idea where you want to head with this and not to waste money on minor diversions. Do not waste money modifying equipment you do not plan to keep in the end. Save and buy the equipment you want permanently and can afford. Then you can deal with minor issues and other problems that might arise (like my turntable's feedback problems). 

@rauliruegas , I prefer arms that are lighter. With lower compliance cartridges you can always add mass. It is much harder to take it away. I never use very low compliance cartridges because they will increase record and stylus wear and generally do not track as well. I go with medium to high compliance cartridges. There are very few ultra high compliance cartridge's available now. Not sure why. Maybe you know. Perhaps it is because nobody makes the ultra light arms they require any more. 

Remember, I own Sound Labs speakers and have been dealing with ESLs since 1978. The X350 will work, probably better than most but the high impedance at low frequencies will stifle it and choke the visceral component important to good bass. With ESLs you have to consider the amp as a component of the speaker. Generally amps with very large power supplies and ultra low impedance output sections will work better. They also have to have very good cooling because of the very low impedance at high frequencies demands high current which produces a lot of heat. The X350 is fine in that regard. Upgrading everything else is important but only applies to records. Improving the amplifier match will make a significant improvement with any source. 

@sheridanmartinj , next trade in the X350 for an XA200.8 and you will make an even larger jump in performance. Do not let the power specs fool you. The XA200.8 is a more powerful amp.

@shinemaster , you forgot Zenith! 

@limomangus , it seems you are a music lover but not an audiophile. Perhaps you are on the wrong site. The vast majority of us can not throw unlimited money at our systems. The trick is making the best system we can afford. Discussion is useful in that regard as well as a lot of research.

@rauliruegas , The combination of the head shell and the jack assembly will never be as light with less inertia as a fixed head. If there were no penalty for removable head shells then every one would have them. The best compromise I think is the Kuzma version. It still requires a heavy steel grub screw to lock the head shell in. SME moved away from removable head shells decades ago. Their best arms continue to have fixed shells and I know you like the SME V series as I do. It is a pity you can't get them any more without buying one of their tables. Hope they reverse that decision. 

@rauliruegas, that poster obviously has not heard them with the right amplifier.

@chungjh , I think your ears then are not used to higher volumes. Start at lower volumes and slowly add 5 dB every few minutes and you will get used to the volume. Sound Labs will make great bas down to 30 Hz or so before cancelation wipes the out. Your not adding subs for bass (unless you really want everything below 30 Hz). You are adding then to relieve the Sound Labs from having to make bass. The result is much less distortion.