Advice on adding a sub to my system

Keep an eye open for the HSU 1220/1225.I believe these offer a crossover/amp and a pair of subs.Many will debate the merits of stereo subs,but it is my preference and is more uniform in its'bass performance.

The SED winged "C" EL-34's are probably the warmest new tubes.
They go for about $120-140 for four? The HSU with the tunable
ports (foam plug for port) may help with inegrating it into your system.VFT-2 series?

Hi, I had a Jolida 502, and used nos Tung Sol 6550's and the best 12at7;'s I heard in this amp were Siemens 2 mica. If you are using the stock tubes, I think you will be able to improve the sound a lot with NOS tubes, especially in the warmth/weight department. I used a sub for a while with the Jolida, and with other amps as well, when I had Spendor sp1/2e"s. I was happy with the sound, but when I got a pair of full range Vandersteen 3a speakers, I realized how much the monitor/sub thing sucked. YMMV. I spent days and days, had test tones and a spl meter, I achieved a pretty flat response +/- 3 db from 30hz to 100hz, but not even close to the Vandersteens. I have since heard only one other system where sub integration was attempted, and IMO it was obvious there was a separate sub is in the mix, I have read that if you are going to attempt this, to get 2 subs, but for 500 bucks this may prove difficult. My James emb-1000 is now used for movies.
My advice if you decide to integrate a sub is that less is more.

If you're going to change tubes I'd suggest first trying to replace the 12AX7s in the Jolida (assuming that you're currently running stock tubes). Power tubes are expensive to replace and must be biased, and besides, I believe the stock tubes in the Jolida are re-branded Valveart EL34Bs which are on the warm side already. For 12AX7 suggestions there are numerous NOS candidates with a warm signature (Mullard, etc.). There may be some new production tubes that will give you what want as well, like the New Sensor Mullard, although I haven't heard too many new production tubes (the Groove Tubes 12AX7M has a warm sound to it, although I'm not sure if it's still in production).

Because tube rolling a pair of 12AX7s can be done for under $100 I'd suggest trying this before adding a sub. If you like what you hear you can always try further tube rolling with EL34s or 12AT7s to maximize your current gear.

Good luck.

Look for a Vandersteen 2W which sell for around $500 used.

Tough to beat at that price.