Yeah I'm serious about the step ! Perhaps more so for Newts on equatorial mounts, I haven't owned a Dob. However I suppose if you were vertically challenged it would be a problem !
The 3x Barlow may be overdoing your magnification a little but could also not be very good quality. I use a 2x SkyWatcher barlow ( not fantasic but not bad ) with my Vixen LV eyepieces 4mm & 20mm. With the 4mm + 2x barlow ( 600x mag ) things start to get a lot out of control as regards sharp image, big image though. With 4mm ( 300x mag ) by itself things are more under control ( my preferred view on clear nights ). With the 20mm + 2x barlow ( 120x mag ) things are sharp as. I read somewhere that's it's much better to have a small sharp view than a large blurr .... and it's true. The main thing with your 12" is the light gathering capability to see fainter objects better not the extra magnification that any extra focal length will give.
Having said all that it depends what you are looking at as well and don't forget the barlow slows down your scope i.e. makes the image darker.