First, the power outputs of the 2 vary significantly. In pentode/ultralinear/beam-tetrode mode, for which both were designed, a pair of '88s is capable of as much as 100 Watts, altho perhaps 70 is more common, while the '34's typical max. per pair is around 50*, with around 40 being more common.
On sound quality, the consensus seems to be that the '34 has more lushness or richness in the midrange than almost any other mid- to hi-power output tube while being a little soft at the frequency extremes. The '88 (and its very close cousin the 6550) is more balanced across the frequency band, exhibiting more-extended and -controlled bass and better-quality treble than the '34 while lacking the '34's (euphonic?) midrange richness. These differences probably can be swamped by the extremes of different brands of the 2; the above is average or typical.
* Conrad-johnson is currently getting 60 Watts per pair of '34s in their MV60, but that has much-larger-than-average (for '34s) output transformers.
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On sound quality, the consensus seems to be that the '34 has more lushness or richness in the midrange than almost any other mid- to hi-power output tube while being a little soft at the frequency extremes. The '88 (and its very close cousin the 6550) is more balanced across the frequency band, exhibiting more-extended and -controlled bass and better-quality treble than the '34 while lacking the '34's (euphonic?) midrange richness. These differences probably can be swamped by the extremes of different brands of the 2; the above is average or typical.
* Conrad-johnson is currently getting 60 Watts per pair of '34s in their MV60, but that has much-larger-than-average (for '34s) output transformers.
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