It’s much easier to replace a cartridge first if you’re not happy with a sound. If your JLTi is mk4 you can send it to Joe for upgrade to MK5 (different power supply), check on his site.
I am the owner of JLTi phono stage and upgraded Zu Audio Druid MK4 speakers. The phono stage is excellent for both MM and 99% of the MC, so no problem with that. Zu audio speakers works fine with both Tube and SS amps, amazing for low power amps, even 5 watt is mothe than enough for my Zu Druis (as they are 101db). I’ve tried various tube amps with rare NOS tunes and ended up with SS ! But you have to try First Watt or Pass Labs with your Zu. Those amps are very special and unique, most of them designed for the speakers like Zu Audio. Each amp made by Nelson Pass is different. I’m using First Watt F2J current source amp with Pass Labs Aleph L 1.2 passive/active preamp (look inside), i also have First Watt B1 buffer preamp as an alternative (this is how it looks inside). I like both, in my opinion his gear is much better than any tube amp i have tried.
Find yourself a better cartridge, because your Denon 103 has conical tip you can not reproduce a good bass and good hights, this is oldschool cartridge with rolled off on both ends. No matter how good is the cartridge body made by Zu (it looks nice, but it has nothing to fo with an awful Conical tip). You have a big disadvantage in your cartridge design, not anywhere else (probably). The reason why people always invest to that Denon much more that it cost by replacing a cantilever and stylus tip for something better.
If you want to use a low compliance MC, but with much better everything you need a stylus like the modern Replicant-100 (read here). Another great low compliance cartridges are Fidelity-Research FR-7f or PMC-3 (both with Refined Contact Line diamonds).
I’ve never heard about your tonearm, but i hope the effective mass is heavy to use such a low compliance like Denon, SPU or FR ? If not, it can be a problem! You have to clearly understand what you arm designed for (low compliance, mid compliance or high compliance cartridges) to choose the cart accordingly.
If the arm is not heavy you can try an MM cartridges (mid of high compliance) like Victor X-1II (Beryllium/Shibata), Stanton SC-100 WOS (Sapphire coated alluminum cantilever with Stereohedron stylus) or Grace F14 and LEVEL II with Boron/MicroRidge styli. Once you will check them out all the pride of owing an MC will be wiped away quickly, you already have an amazing phono stage for MM.
Buying another phono stage you can only understand how good is the JLTi, i have 3 different phono stages, SUTs, Headamps etc. The JLTi is a must have, for its price it is a steal!
I am the owner of JLTi phono stage and upgraded Zu Audio Druid MK4 speakers. The phono stage is excellent for both MM and 99% of the MC, so no problem with that. Zu audio speakers works fine with both Tube and SS amps, amazing for low power amps, even 5 watt is mothe than enough for my Zu Druis (as they are 101db). I’ve tried various tube amps with rare NOS tunes and ended up with SS ! But you have to try First Watt or Pass Labs with your Zu. Those amps are very special and unique, most of them designed for the speakers like Zu Audio. Each amp made by Nelson Pass is different. I’m using First Watt F2J current source amp with Pass Labs Aleph L 1.2 passive/active preamp (look inside), i also have First Watt B1 buffer preamp as an alternative (this is how it looks inside). I like both, in my opinion his gear is much better than any tube amp i have tried.
Find yourself a better cartridge, because your Denon 103 has conical tip you can not reproduce a good bass and good hights, this is oldschool cartridge with rolled off on both ends. No matter how good is the cartridge body made by Zu (it looks nice, but it has nothing to fo with an awful Conical tip). You have a big disadvantage in your cartridge design, not anywhere else (probably). The reason why people always invest to that Denon much more that it cost by replacing a cantilever and stylus tip for something better.
If you want to use a low compliance MC, but with much better everything you need a stylus like the modern Replicant-100 (read here). Another great low compliance cartridges are Fidelity-Research FR-7f or PMC-3 (both with Refined Contact Line diamonds).
I’ve never heard about your tonearm, but i hope the effective mass is heavy to use such a low compliance like Denon, SPU or FR ? If not, it can be a problem! You have to clearly understand what you arm designed for (low compliance, mid compliance or high compliance cartridges) to choose the cart accordingly.
If the arm is not heavy you can try an MM cartridges (mid of high compliance) like Victor X-1II (Beryllium/Shibata), Stanton SC-100 WOS (Sapphire coated alluminum cantilever with Stereohedron stylus) or Grace F14 and LEVEL II with Boron/MicroRidge styli. Once you will check them out all the pride of owing an MC will be wiped away quickly, you already have an amazing phono stage for MM.
Buying another phono stage you can only understand how good is the JLTi, i have 3 different phono stages, SUTs, Headamps etc. The JLTi is a must have, for its price it is a steal!