The little dot is good for getting started and there are enough tube options for the 6J1 (which translates to the 6AK5 or 5654 in US tube designations) that you can get an idea what tube rolling is all about. I'd leave the power tubes alone and try 3 or 4 different pairs of 6AK5s in the front two sockets. Personal choices would be a Raytheon 6AK5, a Western Electric 403b, and a GE 6096 pair. Changing out the front two tubes (the drivers) will make the most difference to the sound and their is considerably more variety available than for the rear two 6NP power tubes and any change made to the front two is then magnified as it passed the rear tubes. You will want a high impedance headphone to test with. I'd see about finding an early AKG 240 or a Beyerdynamic DT990 (either in 600 Ohm form) or a HD6xx, 660, or 800 in the Sennheiser line.