The standard thing to do with a Recto is to throw an overdrive pedal in front of it. The Ibanez Tubescreamer in it's various incarnations is usually a sure bet.
This is pretty much the industry standard when playing metal, and I would go as far as saying that 90% of the metal albums that were tracked with a Recto, were tracked with the OD+Rectifier combination.
The overdrive pedal is not used to bring more gain, but to boost the pre-amp section of the amp and to remove some of the flubby low frequencies from the signal coming from the guitar.
And remember, when using the red channel on modern mode, be careful with the presence control! A lot of people complain that the 3channel Rectos are fizzy, because they are used to use high presence settings and they crank the presence, thus creating a very harsh, thin, fizzy tone.
The red channel has A LOT of presence. because of the very aggressive presence control. I usually have the presence at 9o'clock and the tone is bright even with those settings, but it's more balanced and there's no fizz. The red channel, modern, diodes, bold, pretty low presence settings and a Tubescreamer=crushing metal tone!
The orange channel is darker (similar to the red channel on the older 2channel Recto's! That's what confuses a lot of 2channel Recto players)