Because of the impedance dip of the speaker, the KT-88's were recommended. Why is this the case?
Good question. At first glance it would seem tubes are totally unrelated. Tube amps have different speaker connections for 4 and 8 ohm speakers. But there is no right or wrong. They both come off the same output transformer. The difference is one comes off a transformer tap with more turns, which changes the power ratio of the transformer. Which one is "right" is totally up to you. Use the one that sounds the best.
Same goes for tubes. There are differences between tube types that are easy to hear. KT88 does sound different than 6550C which does sound different than EL34. Within each tube type there are also differences.
Here's where the KT88 advice may be coming from. The sound difference you hear with different speaker impedance can in some ways be balanced out or tuned to your preference by tube selection. There is no right or wrong it all comes down to what you like. So what they are really saying is a flat-out guess that KT88 might be more likely to work in your situation.
The main thing to learn is this is a flat-out guess. They know your amp, and they know the tubes, and they might even know your speakers- but no way they know you or your room or anything else. So KT88 is a good safe bet.
Personally I would forget about impedance and when you get these and listen a while maybe think about how you would like it to sound better. Then if you want a little bit more punch and drive you might try 6550C. If you want a little more liquid midrange warmth EL34. Or whatever else your amp can use. Forget technical stuff like impedance, power, etc. Focus on descriptions of sound quality. Liquidity, soundstage depth, that kind of thing. Those kinds of things tend to hold true regardless of which amp they go in or which speakers are used.