Flat impedance curve?

When building speakers/crossovers, we can regulate the impedance down only, meaning we can stop big peaks, but we cannot stop big dips. The big deal is to have stable impedance around the crossover points so that they are accurate. Rises elsewhere typically don't bother most amps tubed or ss, but as Unsound mentioned, impedance phase angles can deal tubes a fit, especially in lower frequencies at lower impedances. Most designers choose drivers that don't have big impedance dips then stableize rises, then overall, you end up with a well behaved speaker.

Sorry Jwal, I realized that I did not directly answer your question, you will need to post your tubed amp here and a couple tubed amp guru's can chime in... the answer is some tubed amps can handle impedance variences better than others.