Yes, and No is the answer
Thank you for bringing up this question because I always thought that the WT believes in Sola Scriptura but I think it is a combination of both. Allow me to explain and maybe help breakdown the problem with this. I don't believe in Sola Scriptura so I may bring a different perspective on this.
First off, the correct definiton of Sola Scriptura is:
Sola scriptura (Latin ablative, "by scripture alone") is the doctrine that the Bible is the only infallible and inerrant authority for Christian faith, and that it contains all knowledge necessary for salvation and holiness. ...
The essence of that definition would mean that the WT believes the Bible is the foundation for its reasoning and authority. There is nothing in the above definition that one would need another person to understand it or not. Some do and some don't. It's not an issue with the definition. We need not cloud the issue here with that. The Bible is claimed as the sole basis for the WT's authority given their interpretation in Mathew on the faithful slave.
On this basis they do believe in Sola Scriptura. In other words no other source points to them as being God directed except the Bible, based on their own interpretation. No tradition, nothing else. They only use the Bible to identify themselves. Therefore, only Sola Scriptura for them.
In contrast to Sola Scriptura:
Q. What is tradition?[pg. 87]
A. The doctrines which the Apostles taught by word of mouth, and which have descended through every successive generation even to our times.
Source: A Doctrinal Catechism, by the Rev. Stephen Keenan, published in 1848 by E. Dunigan & Brother, [James B. Kirker], in New York, No. 371 Broadway, pgs. 86-87.
This is the contrast of Sola Scriptura. The idea is that the Bible alone is not our only authority but that the Apostles handed down Holy Traditions that are included in pure worship. Traditions that are also necessary to understand doctrine, and the Bible, and pure worship.
The WT does not accept this concept. Yet in a twist of things they include themselves to be necessary, as in Holy Tradition, in order to understand the Bible.Therefore on this basis they do not accept Solo Scriptura for the faithful. The only accept this concept only for themselves, as an organisation.
So like usuall the WT uses both concepts twisted up to become an image unto themselves. First they say that they believe only in the Bible and not Traditions and they also say that you must go to them to learn what the Bible has to say. They have set themselves up to be the tradition, and doctrine, without saying it outright.
They become the authority verses the traditions handed down from the apostles, (speaking from a Catholic perspective).
So here is the deal. They believe in Sola Scriptura only on the basis that it points to them. They believe in tradition or some authority that gets handed down to others because the Sola Scriptura only points to them and the faithful must understand them to understand the Bible.
While most Protestant churches teach the Sola Scriptura concept, and is in fact the basis for there belief that one can read the Bible on their own and gain knowledge, they don't go beyond this and make themselves to be THE authority to go through in order to get there. Therein lies the difference.
A Protestant church may say they can help you understand the Bible or a person may see truth in there teachings, but they are always free to choose or leave and move elsewhere because most Prostestants don't hold the church to be guiding them only the Holy Sprit and the Bible.
The difference in simple terms is a Protestant looks to the church as an aid to help them understand just like the Ethiopian did with Paul, but don't believe the church is a kiosk needed to go through before things can be understood.