Upgrade from Technics SP25

I am looking for an upgrade that digs a little deeper and extracts more detail.

No wonder, your conical DL-103 diamond simply missing the extensions on both ends due to natural limitation of the contact surface associated with any conical/spherical profile. This is very well explained here.

If you want to keep using Low Compliance cartridges with your tonearm (which must be high mass) then you have to upgrade your cartridge first. If you’re in love with the Denon then all you need is to replace the whole cantilever/stylus on this cartridge to something modern such as Ruby/LineContact which you can do with a help of any retipper such as SoundSmith or many others (for the cost of lower than new Hanna MC).

If you still looking for something fresh and another cartridge is what you need then you can still buy a much better cartridge for your tonearm to try first. I would recommend Fidelity-Research PMC-3 low compliance MC cartridge with nude Line Contact diamond, Air Core Coil ! If you can’t find it just ask me.

Another decent upgrade for the Denon 103 aficionado is definitely the Ortofon SPU Royal G mkII with the most complicated profile in the history - The Ortofon Replicant 100. With this profile you will extract maximum from your records. Also no need to change the arm if it can support about 30g cartridge/shell. Forgive me, but i will link my listing, so you can read amore about it.

Doing my research for the best low compliance cartridge i ended up with mega rare FR-7fz, but i think this cartridge alone exceed your budget.

However, FR cartridges and SPU cartridges are all voiced with alluminum cantilever (and nude pressure fitted line contact type diamonds) and does not require an exotic cantilevers to sound absolutely fantastic. That means the service of those cartridges is much cheaper, even if you need a brand new cantilever with new tip, any retipper can do that for you for extremely low price, compared to the exotic cantilevers like Boron.  

In my opinion you have a nice DD turntable, if your plinth is fine then i would definitely recommend to start with a new cartridge, the DL-103 is the weakest part on your tunrtable if you’re looking for much more detailed sound.

If you need a mid or high compliance cartridge then it’s another story and you could try an MI or MM, but you have to check for tonearm effective moving mass with the lightweight headshell/cartridge to avoid undesirable resonance.

There was an SP-20 released in 1976 which is exactly the same size as the SP-10 mkII, but without external power supply. Actually very nice turntable, i have posted about it on audiogon few years ago.  

I know it's fun to change the turntable, i did it many times, nothing can beat the DD Luxman PD-444 in terms of usability if the owner would like to swap the tonearms. 

In my opinion changing the SP-25 to something else will not change the sound if the tonearm and cartridge are the same. SP-25 is a nice DD with stable rotarion and high torque, if the plinth is nice then it is a great combo. 

The most important is the cartridge and sinergy between the cartridge and tonearm. Definitely the phono stage too. Turntable just rotating a platter with a record on it. 

I've owned two SP-20 and one SP-10 mkII. You can upgrade your SP-25 with a nice air craft alloy mat from SAEC, the SS-300 is a must have between the LOMC cartridge/record and the platter if the motor is right under the platter. Micro Seiki CU-500 is even better.