800's have much more gain than 900's. I used to have a 900, sold it to buy a 800. I have a 1987 50 watt head, 2 channel....
The 4104 is the combo version, right?
1 or 2 channel? (can't remember what year they introduced the 2 channel model)
Does it have a master volume?

The guitars may have low output pickups, which will make a huge difference. The SG I used to have had low output pickups and I never could get as much gain as I wanted with that guitar....My LP Classic has really hot pickups and I get all the gain I need, actually turn the gain down to around 8. I also just put Seymour Duncan Blackout active pickups in my Ibanez and I have to turn the gain down to around 7.....

Gain on 10 with low output pickups is about the same as a gain setting of 6-7 with average high output pickups....just an observation I've made....

Also, an easy mod for an amp tech is to add an extra pre-amp tube. I've played through a modified 800 like this and it melted my face.....but in all honesty, this mod isn't really needed....more gain can be found in other ways.

As far as pre-amp tubes themselves, different tubes can make the difference. I just recently replaced my pre-amp tubes and found that only 2 of them provided the gain I want. So naturally, I put those 2 in the first 2 spots (2 closest to the power tubes). One tube I have is really noisy (microphonic). Microphonic tubes will not sound good in the 1st, 2nd or 5th position (at least on my amp) as those tubes make a lot of weird noises. For whatever reason, if that microphonic tube is in the 3rd or 4th spot, you can't hear it (One of these positions powers the eq I believe). The first pre-amp tube position (one closest to the power tubes) is the main tube for the overdrive. You can actually try just swapping around the pre-amp tubes that are in it now....but be careful, you can get shocked doing this....just be sure to turn the amp off with the standby switch still on to drain all the power out of the amp....It is possible to get shocked even with the amp unplugged, so be sure to drain that power.....

As far as tube brands, I recommend Tung-Sol.

Here's a website to check out that will explain the characteristics of different tube brands and types....

http://thetubestore.com