Throttle Box or Tube Screamer?

I'm looking to smooth out the gain and add some sustain to my Dual Rectifier.
Looking for more of a "metal" sound.

Would either a Throttle Box or Tube Screamer work?
If so, which would be best?

The Throttle Box is much higher gain than a Tube Screamer - its more in the range of Recto high-gain. You may be more interested in something like the Grid Slammer which is similar to the TS - or the Flux Drive which is a step above the drive from a TS, but with the independent Bass and Treble controls. Very versatile! Thanks!

+1 on the Grid Slammer. I've got a Grid Slammer running into my Mini Rec. I use it to boost a slightly crunchy "pushed" mode or to get just a bit of hair from "clean". It's got a subtle mid hump that adds just enough to rise above the mix.

I'm running my DR on "Bold / Silicon Diode".
It sounds best (to my ears) with these settings but I'm still unable to smooth out the gain.
I checked the tubes and found that it came with EL34's. This makes sense as it sounds a lot like a Marshall. Lots of punch and nice crunch. However, this doesn't always work well with metal (imo).

I know this is probably not the section to ask this question but would switching out the EL34's for 6L6's help smooth out the gain and make it tighter?

I really prefer NOT to have to use any sort of OD pedal with this amp.