During the last few years I have gradually worked my way through the GNW volumes of LVVA, fond 7349 - the Riga part of the archive of the Governor General of Livonia. Recently I decided to take a look at some of the volumes from his colleague in Estonia, preserved in the Estonian Historical Archive in Tartu as fond 1. These records have, generally speaking, fared better than the Livonian ones, but certain problems derive from the fact that they were evacuated to Sweden before the Russian conquest in 1709-10 and only partly returned after 1721. This means for example that a large number of letters from Henning Rudolf Horn in Narva to Governor General de la Gardie in Reval from the period 1702-1704 are preserved in Stockholm, while most of the letters from Governor Vellingk to de la Gardie during the campaign of 1700 appear to be in Tartu. The latter were used by Hendrik Sepp for his study Narva piiramine ja lahing a. 1700 (1930), but as far as I know not by any Swedish historian.
At this point I have only looked at volumes EAA 1.2.284-286, which contain incoming correspondence for 1700 and the first seven months of 1701. It's quite obvious that these only contain a very limited part of incoming letters, predominantely those from Vellingk, the garrison commanders at Pernau and Dorpat as well as some from Dahlbergh in Riga, County Governors Lindehielm in Vyborg and Örnekloo in Arensburg and various local commanders.