I'd go for a combination of volume boost and EQ, and if your Effects / Amp has presets then it's easy. Stick with your volume difference, but on the solo preset as suggested boost the mids and the treble, and cut some of the bass, the key to most lead tones is that 'raspier' tone, not full on scooped mids. Also, as a suggestion, try dialling back the Dist in your rhythm tone (I don't know what setting you are using but if u are using full distortion try rolling off to say 7 or 8).

If you want to go even further, if your distortion presets have a master volume setting (NOT your amp's volume setting) then you can use that to set a volume for your rhythm sound, and a different, higher one for your solo. but definitely use EQ as well as volume when creating a lead tone!

I used to be set up exactly as you are with the channel volumes set lower on the rhythm channels and then maxed out on lead channel.  It really wastes a channel on your bank.  I just got the Spider IV 150 combo and MKII Shortboard.  The Spider IV's have a boost setting on every channel and a button on the MKII to activate it.  You can set it in .5 dbl increments.  I was surprised to find it but it's an awesome feature.  So happy with my Spider IV.