Okay, now we got a starting point.
Both companies have done a lot of work on output transformers since the late 90's and those really dictate the sound. The newer Neo-classics are more than just an evolution just as the Siegfrieds are for VTL.
Before that, differences are much less than switching to triode. The reason for that is that both companies have tried to remain consistent with David Manley's origional sound (just the right amount of distortion). Both are optimized for 5 ohm loads and both aren't followers of the lush, euphonic tube sound although they do sound more "organic" than solid state. I would describe the soundstages similar as well, being both wide and slightly forward but stable. In the heavyweight class, both have good punch, espescially later versions. The Neo's might have a bit more finesse while the Signatures are a little more dynamic. It's like comparing 6550's to KT88's. If I'm sounding vague, it's because I've never had the opportunity to compare them head to head or even in the same system. There are so many variables with tubes that heresay is useless.
Earlier Manley's had slope/feedback controls that allowed tailoring to the speakers. Before that and the split, VTL did all the commercial designs while Manley focused on pro gear. Still, the bulk of Manley's income is from studio equipment but has become diversified. There are some interesting older pieces like the Ichibans which were designed specifically for triode operation.
Either VTL or Manley would be a good choice for IRS speakers and just be happy that you have a wonderful system. Both have enviable service records and support.