Stress on Output Tubes

I have a Music Man HD-130 bass version and am running 4 Ruby EL34 output tubes.
I would like to try the amp with an extension speaker(s)at 4ohms.Will this make the amp work harder and cause trouble for the EL34s being their plate voltage rating is around 450v?
I do run on high power due to the nature of the bass guitar overdriving the preamp(12ax7).

It's really causing no extra strain but I would make sure that you have the load switch in the proper spot. That MM amp puts the output jacks in series so the cabinets ADD their loads. In other words, if you use the secondary jack, two four ohm cabinets become eight ohmm, two eight ohm cabs become sixteen ohms, etc. If you use the jacks in the cabinets to "daisy chain" them, two four ohms becomes two ohms, two eight ohms becomes four ohms, etc. You have to pick your combination and set the switch on the back appropriately. Mike.

Ok to get to 4 ohms I should be daisy chaining cabinet to cabinet with the switch at 4ohms.
Now....I also have a 4ohm cabinet and an 8 ohm.Where is that going to put the impedence?Is it safe for the amp? Both cabs have 2X10s.
Thanks Mike!!!

Sorry I haven't answered before-I didn't see this post! Anyway, your speaker load in parallel would be 2.66 ohms which is right at the bottom of where you could get away with on the four ohm setting. I, personally, would use the series jacks on the back of tha amp and that would put the two speakers in series and give you a 12 ohm load and put the amp on 8 ohms. Old way of figuring two mismatched loads is product over sum. In other words, 4x8 ohms= 32. 4+8=12. 32 divided by 12 equals 2.66. Mike

Thanks.....appreciate the response!