Siemens EL34s are really nice, very crunchy and aggressive. They're not like Mullards though, Mullards just have 'IT' - fat, warm and crunchy all in perfect measure. I have two of each in my 100w HTH high gainer, best of both worlds.
I will say, I can never get a plexi to sound right unless it has Mullard EL34s in it - those amps just sound too thin otherwise. As always, ymmv etc.
With higher gain amps, Siemens are great - I had a 2203 from '81 that didn't sound right unless it had those EL34s in it.