I own a pair of mint Model 19's and a pair of Valencias as well as a pair of Heathkit AS-101's (Valencia drivers with a Heathkit crossover and cabinet). The Model 19's are my newest purchase (fall 2009) and they are the best sounding speakers I have ever heard--bar none. My Valencias and AS 101's sound almost as good with equalization from my Alesis EQ, but the Model 19's sound amazing with minimal EQ. I'm seriously considering upgrading my Valencias (and possibly the 101's as well) by swapping out the 806's with GPA 902's but not sure if the stock 800 hz Valencia crossover will work?? I went to the Altec pro site and looked up the specs for the 806 (stock Valencia) h.f. driver. It shows the pressure/sensitivity (i.e. efficiency) is 109.5 db at one watt input. The pressure/sensitivity for the 902 is only 104 db (105.7 on the GPA website) at one watt input which makes me think that the 902 wouldn't be "loud" enough to replace an 806 using the stock Valencia crossover--I'm guessing I would need a model 19 crossover to compensate for the difference in high frequency driver efficiency if I swap out two 806's for two 902's? Am I right in assuming that at the same power level the 806 will be "louder" than a 902 or are they comparable efficiency and it's just that they were measured differently which caused the difference between 109.5 db and 105.7 db?? Besides the efficiency factor, another factor to consider is that I've heard that the 902's don't sound good with a 500hz crossover point--would they be fine with an 800hz crossover point (stock Valencia) or do I need the 1200 hz crossover point that a Model 19 crossover provides? Also, in the same vein, the Model 19's use 416-8B woofers. The AS 101's use 416-8A woofers. Are they going to be comparable or do I disqualify the Model 19 crossover for a Valencia/AS-101 upgrade because it was designed for 416-8B's and not for 416-8A's?? And lastly, the Altec 902 is rated at 104 db with one watt input, the GPA 902 is rated at 105.7 db at one watt input. Again, do I disqualify the model 19 crossover for this upgrade because it's specifically designed for use with the Altec 902, or are the Altec 902 and the GPA 902 comparable and just measured differently (one with current measuring technology and one with older measuring technology?)??? It would be nice if I could just use the stock Valencia (and AS 101) crossover (800hz) but I don't want what's easy (and less expensive), if I do this I want to do it right..... It just occurred to me that since the cabinets are different for a Valencia than a Model 19, does that disqualify the Model 19 crossover as a candidate? Are there enough questions posed in this post???