[Kreisler jr]

From this text it seem quite clear that Tchaikovsky has a very different position from Wagner.
Because Wagner would obviously never have agreed that his works went downhill after Lohengrin (and most Wagnerians would say, the best ones start past Lohengrin...). Neither would Wagner have agreed that symphony and opera were opposed. A central point of "Gesamtkunstwerk", Wagnerian musical drama, is that this is the continuation and culmination of BOTH pre-Wagnerian symphony and opera.

Most would agree that Wagner's operas are uncommonly "symphonic". And I would also expect him to be able to write more convincing symphonies than e.g. Verdi or even Tchaikovsky (he wrote good symphonies but they seem rather clumsy as "absolute dramatic music" with their oscillation between programmes, isolated operatic gestures and classicist preciosity).

But from this is hardly follows that Wagner's symphonies would have been better than Beethoven's (or Brahms' or Bruckners) and especially not that they would have been higher achievements than his operas. Maybe they would have been great, but it's pure speculation with little basis. I don't know which other composer wrote the "most Wagnerian" symphonies... any candidates?

 

[Kreisler jr]

These two were (mostly) pre-Wagner, certainly before full late Wagner leitmotiv technique. I was thinking more of post-Wagnerian pieces with clear influence from 1860s-70s Wagner.
IMO Berlioz and Liszt both wrote some good music but they were far more inconsistent than Wagner (or Beethoven, or "conservative" symphonists like Brahms) and their symphonies/tone poems give me no good clue how good symphonies by Wagner or someone following Wagner would have sounded like. (It has been claimed that what might be Berlioz' best instrumental piece, the Love scene from R&J is the most Wagnerian, prefiguring Tristan. Maybe, but most Berlioz to me does not seem like that.)

Of course, I don't assume that Wagner would have written conventional symphonies (tbh I think Berlioz' are mostly quite conventional). But I am not sure whose tone poems I'd perceive as properly "Wagnerian". Admittedly, I don't know too many tone poems all that well, generally not that fond of the genre (Wagner's orchestral preludes/interludes are usually better than most tone poems...)

The Siegfried Idyll has been mentioned; I have to admit that I find this a bit boring, in any case it is a mostly undramatic lyrical, seamless unfolding. Imagining a "Siegfried symphony" twice as long puts me too sleep only thinking about it.